Analog regulators based on operational amplifiers. PWM controller on an operational amplifier

The controller calculates the error (the difference between the reference signal and the feedback signal) and converts it into a control action in accordance with a certain mathematical operation.

In ACS, the following types of regulators are mainly used: proportional (P), integral (I) and proportional-integral (PI). Depending on the type of signals being converted, analog and digital controllers are distinguished.

Analog regulators(AP) are implemented on the basis of operational amplifiers, digital- based on specialized computing devices or microprocessors. Analog controllers only convert analog signals that are continuous functions of time. When passing through the AR, each instantaneous value of the continuous signal is converted.

To implement the AR, the operational amplifier (op-amp) is switched on according to the summing amplifier circuit with negative feedback. The type of regulator and its transfer function are determined by the connection circuit of resistors and capacitors in the circuits at the input and in the feedback of the OS.

A proportional regulator (P-regulator) is implemented when a resistor with resistance R OS is included in the feedback circuit of the op-amp. This regulator is characterized by a proportionality factor to , which can be greater than or less than one.

The integral regulator (I-regulator) is implemented when a capacitor C os is included in the feedback circuit of the OS. This type of controller is characterized by a time constant T.

A proportional-integral controller (PI controller) is implemented by including a resistor with resistance R os and a capacitor C os in the feedback circuit of the op-amp. Such a regulator is characterized by the following parameters: proportionality coefficient to and time constant T.

For all types of regulators in the implementation circuit, there is an input resistance R 1.

Schemes for the implementation of regulators, the dependence of the voltage at the output of the regulator U out on the input U in and their graphical representation, as well as formulas for finding the parameters of the regulators are shown in table 1

Table 1 - Regulators

Explain what current sensors are for, what requirements are placed on them. Give functional diagrams of a DC electric drive with a transformer current sensor and a current sensor based on a shunt.

Current sensors (DT) are designed to obtain information about the strength and direction of the motor current. They are subject to the following requirements:

Linearity of the control characteristic in the range from 0.1I nom to 5I nom not less than 0.9;

Availability of galvanic isolation of the power circuit and control system;

High performance.


The AED coordinate sensor can be structurally represented as a serial connection of a measuring transducer (MT) and a matching device (CU) (Figure 1). The measuring transducer converts the coordinate X into an electrical voltage signal And(or current i), proportional X. The matching device converts the output signal And IP into the feedback signal u os, which satisfies the ACS in size and shape.

Figure 1 - Structural diagram of the AED coordinate sensor

Current transformers, additional (compensation) windings of smoothing chokes, Hall elements, shunts are used as measuring transducers in diesel fuel.

Current sensors based on shunts are widely used for measuring motor current. Shunt is a four-terminal resistor with a purely active resistance R w(non-inductive shunt), to the current terminals of which the power circuit is connected, and to the potential - measuring. (picture 2)

To weaken the influence of the shunt on the passage of current in the motor circuit, its resistance should be minimal. The nominal voltage drop across the shunt is usually 75 mV, so it must be amplified with an amplifier U. Since the shunt has a potential connection with the power circuit, the current sensor must contain a galvanic isolation device (UGR). Transformer and optoelectronic devices are used as such devices.

Figure 2 - Scheme of switching on a current sensor based on a shunt

DT based on current transformers are mainly used in DC AEDs to measure the current of motors when they are powered by symmetrical bridge single-phase and three-phase rectifiers. For a single-phase rectifier (Figure 3), one current transformer (TA1) is used, and for a three-phase one, three transformers are included in the star. To ensure the operating mode of current transformers close to the short circuit mode, their secondary windings are loaded with low-resistance resistors RCT (0.2 ... 1.0 Ohm). The conversion of the alternating voltage of the secondary windings is carried out by a rectifier VD1 ... VD4.

Figure 2 - Scheme of switching on a current sensor based on a current transformer

13. Give a functional diagram of the armature EMF sensor, explain the principle of its operation.

With low requirements for the speed control range (up to 50), EMF feedback is used as the main feedback in the electric drive. The principle of operation of the armature EMF sensor is based on the calculation of the motor EMF.


The functional diagram of the EMF sensor is shown in Figure 1.

Figure 1 - Functional diagram of the armature EMF sensor

To measure the armature voltage, a divider on resistors R2, R3 is used. To measure the armature current of the motor, an additional winding L1.2 of the smoothing choke is used. Voltage and I through the divider, RC filter and repeater A1 is fed to the adder A2. A signal proportional to the voltage drop across the armature winding is also fed to the input of the adder A2 R i. c ∙i i.

Output voltage expression u de amplifier A2 for steady state operation has the form

where to de is the transfer coefficient of the EMF sensor,

e I am the emf of the armature.


To obtain a signal proportional to the motor armature voltage, a resistive voltage divider can also be turned on according to the following scheme

Figure 2 - Scheme of switching on the voltage sensor

The output voltage of the divider is

The voltage sensor, in addition to the divider, may also contain galvanic isolation devices and

amplifier.

14. Draw a diagram of a vertical single-channel pulse-phase control system, explain the principle of its operation using timing diagrams.

To control the thyristors of the rectifier, a pulse-phase control system (SIPC) is used, which performs the following functions:

Determining the points in time at which certain specific thyristors should open; these moments of time are set by the control signal that comes from the output of the ACS to the input of the SIFU;

Formation of opening pulses transmitted at the right time to the control electrodes of thyristors and having the required amplitude, power and duration.

Consider the operation of a vertical single-channel SIFU controlling thyristors of a single-phase bridge rectifier (Figure 1).

Figure 1 - Scheme of a single-phase bridge rectifier

The alternating voltage generator of the GPN starts when voltage is received from the synchronizer C (Figure 2). This happens at the moment when a direct voltage is applied to the thyristors, i.e. at natural switching points.

Figure 2 - Scheme of a vertical single-channel SIFU

From the GPN output, the sawtooth voltage is supplied to the US comparison device, where it is compared with the control voltage U y (Figure 3). At the moment of equality of the sawtooth and control voltages, the US generates a pulse that, through the pulse distributor RI, goes to the pulse shaper FI1 or FI2 and then through the output shaper VF1 or VF2 to the rectifier thyristors. The output shapers amplify the opening pulses in terms of power and potentially separate the SIFU from the power section. A comparator based on an operational amplifier is used as the US.

Figure 3 - Diagrams of the work of the SIFU

15. Give a functional diagram of an electric drive with a three-phase zero reversing rectifier with joint control and explain the principle of its operation.

With joint control of thyristor sets, opening pulses are simultaneously applied to both sets of VS1, VS2, VS3 and VS4, VS5, VS6 (Figure 1). At the same time, depending on the direction of rotation of the motor, one set operates in rectifier mode, and the other in inverter mode. The armature current flows through the set operating in the rectifier mode.

Figure 1 - Joint control of sets of valves of a three-phase zero

reversing rectifier

The rectifier thyristor control system contains two SIFU (SIFU1, SIFU2) and an analog inverter A1.

If VS1, VS2, VS3 work in rectifier mode, and VS4, VS5, VS6 in inverter mode, then the motor rotates forward. Otherwise, the motor rotates backwards.

Since the opening pulses are applied to both sets, then in the circuit through two open valves, for example VS1 and VS6, a closed circuit of two phases of the secondary winding of the transformer TV1 is formed.

In this circuit, the sum of the EMF of the two phases of the secondary winding operates, which is called equalizing EMF:

where e 1, e 2 - rectified EMF of sets VS1...VS3 and VS4...VS6, respectively.

Equalizing EMF e ur creates an equalizing current I ur. In relation to the circulating current, the TV1 transformer is in short circuit mode, because the active and inductive resistances of the transformer are small. Therefore, to limit the surge current, surge reactors L1 and L2 are included in its flow circuit.

In addition to switching on the surge reactors, the surge current is limited by the coordinated control of the sets, in which the constant component of the surge EMF E ur equals zero, i.e.

E ur \u003d E 1 + E 2 \u003d E 0 (cosα 1 + cosα 2) \u003d 0, (1)

where E 1, E 2- constant components of the EMF e 1 and e 2 respectively; E 0- constant component of the rectified EMF at α = 0; α 1 , α 2 - opening angles of sets VS1...VS3 and VS4... VS6.

Condition (1) will be fulfilled when a 1 + a 2 =p. This condition is a condition for the coordinated control of thyristor sets.

Co-management has the following benefits:

· Balancing currents ensure the conductive state of both sets, regardless of the magnitude of the load current of the motors and, as a result, the linearity of the characteristics (there is no intermittent current mode).

· High speed, due to the constant readiness for current reversal, which is not associated with any switching in the circuit.

However, with joint control, it is necessary to install surge reactors, which increases the mass, cost and dimensions of the electric drive. The flow of circulating currents increases the load on the elements of the power circuit and reduces the efficiency of the rectifier.

16. Draw a block diagram of an electric drive with a reversing rectifier with separate control and explain the principle of its operation.

In a reversible rectifier with separate control, when one set of thyristors is operating in rectifier or inverter mode, the other set is completely out of operation (opening pulses are removed). As a consequence, there is no surge current loop, eliminating the need for surge reactors.

The block diagram of the electric drive with a reversing rectifier with separate control (RVR) is shown in Figure 1. The operation of the RVR is provided by additional elements of the thyristor control system: valve conductivity sensor (DPV), logical switching device (LPU), characteristic switch (RC).


Figure 1 - Structural diagram of an electric drive with a reversible rectifier

with separate control

DPV is designed to determine the state (open or closed) of the rectifier thyristors and generate a signal about their blocking, which is equivalent to the absence of current in the sets.

LPU performs the following functions:

Selects the desired set of valves "Forward" or "Back" (KV "V" or KV "N"), depending on the required direction of the motor current, set by the signal U zt

Prohibits the appearance of opening pulses simultaneously in both sets of thyristors using the keys "Forward" ("B") and "Back" ("H");

Prohibits the supply of opening pulses to the set that is entering into operation until the current passes in the previously working set;

Forms a temporary pause between the moment of closing all thyristors of the previously operating set and the moment of supply of opening pulses to the set that is starting to work.

The characteristic switch serves to match the unipolar adjustment characteristic of the SIFU α = ƒ(u y) with the reversible signal U y.

Reversing the motor begins with a change in the sign of the speed reference, which causes a change in the sign of the current reference U zt. This leads to a decrease in the control voltage U y, an increase in the opening angle α 1 of the thyristors of the “Forward” valve set, therefore, a decrease in the EMF E 1 and, as a result, a decrease in the armature current to zero. The closing of the valves is fixed by the DPV. Upon receiving a signal from the DPV, the LPU prohibits the supply of pulses to the thyristors of both sets (opens "B") and simultaneously starts counting the temporary pause. After its completion, the LPU generates permission to supply opening pulses to the thyristors of the “Back” valve set (“N” closes) and switching the PH. Switching PH leads to a change in the polarity of the control voltage U y at the input of the SIFU. From this moment, the opening pulse with the angle α 2 starts to be applied to the HF "N", ensuring the operation of the set in the inverter mode. Since the EMF of rotation is greater than E 2, the armature current flows in the opposite direction. The engine switches to generator mode, performing regenerative braking.

Separate control has the following advantages:

There are no surge reactors, which significantly reduces the size, weight and cost of the reversible rectifier;

There is no circulating current, which reduces power losses in the rectifier and increases its efficiency.

The disadvantages of the separate equation are:

The presence of an intermittent current mode, which requires linearization of the rectifier control characteristics;

A more complex management system due to the presence of health facilities, WPVs and PH;

The presence of a dead pause when switching sets.

Give and describe the closed structures of the EP built on the principle of compensation for external disturbances and the principle of deviation. Draw a block diagram of a two-loop system of subordinate control of a DC electric drive and describe its blocks.

Closed structural EAs are built according to the principle of compensation for external disturbances and the principle of deviation, also called the feedback principle.

Let us consider the principle of compensation using the example of compensation for the most characteristic external perturbation of the electric drive - the load moment Ms when adjusting its speed ω (Figure 1a).

Figure 1 - Closed EP structures

The main feature of such a closed structure of the EA is the presence of a circuit through which a signal proportional to the load torque is applied to the input of the EA together with the speed control signal Ucs

Um = Km∙Ms, where Km is the coefficient of proportionality.

As a result, the EP is controlled by the total signal U ∆ , which, automatically changing with fluctuations in the load torque, ensures that the speed is maintained at a given level. Despite the efficiency, the EA control according to this scheme is rarely carried out, due to the lack of simple and reliable sensors of the load moment Ms.

Therefore, in most closed circuits, the principle of deviation is used, which is characterized by the presence of a feedback circuit connecting the EA output to its input. In this case, when controlling the speed, a speed feedback circuit is used (Figure 1b), according to which information about the current speed value (signal Uos=Kos ∙ ω) is fed to the EA input, where it is subtracted from the speed reference signal Uss. The control is carried out by a deviation signal U ∆ =Uzs-Uos (it is also called a mismatch or error signal), which, if the speed differs from the set one, automatically changes accordingly and, with the help of the ACS, eliminates these deviations.

Depending on the type of adjustable coordinate, the EP uses feedback on speed, position, current, magnetic flux, voltage, EMF.

The system of subordinate regulation.

To control the movement of the EUT, sometimes it is required to adjust several coordinates of the EA. For example, current (torque) and speed. In this case, closed EAs are performed according to the scheme with subordinate coordinate control.

Figure 2 - Structural diagram of a two-loop slave control system

In this scheme, the regulation of each coordinate is carried out by its own controllers (current RT and speed RS), which, together with the corresponding feedbacks with the coefficients K oss and K oss, form closed loops. These circuits are located in such a way that the input (setting) signal for the current circuit Uzt is the output signal of the speed circuit external to it. Thus, the internal current loop will be subordinated to the external speed loop - the main adjustable coordinate of the EA. The signal U ∆ from the output of the RT is fed to the thyristor converter TP. The EM electric motor is represented by two parts: electrical (ESD) and mechanical (MChD).

The main advantage of such a scheme is the possibility of optimal adjustment of the regulation of each coordinate. In addition, the subordination of the current loop to the speed loop makes it possible to simplify the process of limiting the current and torque, for which it is only necessary to maintain the signal at the output of the speed controller (reference signal) of the current level at an appropriate level.

Explain what static frequency converters with an intermediate DC link (SFC CRCT) are designed for. Give the block diagrams of the CFC CRPT, which differ in the way of regulating the voltage on the stator of the IM.

SFC PZPT are designed to convert alternating voltage with constant amplitude and frequency into alternating voltage with adjustable amplitude and frequency.

There are three types of SFC CRPT, depending on the method of voltage regulation:

1. SFC PZPT with a controlled rectifier

In this circuit, the voltage amplitude is regulated at the output of the rectifier (Figure 1).

Figure 1 - SFC PZPT with a controlled rectifier

SW - controlled rectifier, converts AC energy into DC energy.

F - filter, serves to smooth the ripple of current and voltage.

I - inverter, serves to convert direct current into alternating current.

SUV - rectifier control system.

IMS - inverter control system.

FP - functional converter, is used to convert the frequency reference signal U c. f. in the voltage reference signal U c. u . depending on the implemented law of frequency control.

Depending on the type of filter Ф in the DC link, the autonomous inverter And is divided into AI current and AI voltage. In an AI-based FH, the filter is a reactor L with a large inductance (Figure 2a). Such an inverter is a current source, therefore, in this circuit, the control action on the motor is the frequency and stator current.

Figure 2 - Filter schemes

AI voltage is a voltage source, for which the filter, in addition to the inductance L, contains a large capacitor C (Figure 2b). The control action on the motor in the FFS system with AI voltage is the amplitude and frequency of the voltage.

Fig. 2. SFC PZPT with an uncontrolled rectifier and a converter with pulse-width control (PWIC) in the DC link (Figure 3).

Figure 3 - SFC PZPT with uncontrolled rectifier and PSHIU

In this case, the voltage regulation is carried out in the PSHIM, which is installed between the uncontrolled HV rectifier and the I inverter. The unregulated DC voltage from the HV is supplied to the PSHIM, where it is regulated in value, being converted into a sequence of rectangular pulses, filtered by the F filter and fed to the input of the I inverter.

3. SFC PZPT with an uncontrolled rectifier and with pulse-width modulation of the voltage in the inverter (Figure 4).

Figure 4 - SFC PZPT voltage pulse-width modulation in the inverter

In this circuit, the regulation of the voltage amplitude and frequency is combined in I. Pulse-width modulation is achieved using a complex gate switching algorithm and can only be implemented in converters with controlled keys: with power transistors or with artificially switched thyristors.

The advantages of PWM controllers using operational amplifiers are that almost any op-amp can be used (in a typical switching circuit, of course).

The output effective voltage level is regulated by changing the voltage level at the non-inverting input of the op amp, which allows the circuit to be used as constituent part various voltage and current regulators, as well as circuits with smooth ignition and extinguishing of incandescent lamps.
Scheme easy to repeat, does not contain rare elements, and with serviceable elements, it starts working immediately, without adjustment. The power field effect transistor is selected according to the load current, but to reduce the thermal power dissipation, it is desirable to use transistors designed for high current, because. they have the least open resistance.
The radiator area for a field effect transistor is completely determined by the choice of its type and load current. If the circuit will be used to regulate the voltage in the on-board networks + 24V, to prevent breakdown of the gate of the field-effect transistor, between the collector of the transistor VT1 and shutter VT2 a 1K resistor should be included, and a resistor R6 shunt with any suitable 15 V zener diode, the rest of the circuit elements do not change.

In all previously considered circuits, as a power field-effect transistor, n- channel transistors, as the most common and having the best characteristics.

If it is required to regulate the voltage at the load, one of the outputs of which is connected to "mass", then circuits are used in which n- the channel field-effect transistor is connected by the drain to the + of the power source, and the load is turned on in the source circuit.

To ensure the full opening of the field-effect transistor, the control circuit must contain a node for increasing the voltage in the gate control circuits up to 27 - 30 V, as is done in specialized microcircuits U 6 080B ... U6084B , L9610, L9611 , then between the gate and the source there will be a voltage of at least 15 V. If the load current does not exceed 10A, you can use power field p - channel transistors, the range of which is much narrower due to technological reasons. The type of transistor also changes in the circuit VT1 , and the control characteristic R7 changes to reverse. If for the first circuit an increase in the control voltage (the variable resistor slider moves to the "+" of the power source) causes a decrease in the output voltage at the load, then for the second circuit this dependence is inverse. If a specific circuit requires an inverse dependence of the output voltage on the input voltage from the original, then in the circuits it is necessary to change the structure of transistors VT1, i.e. transistor VT1 in the first circuit, you need to connect as VT1 the second scheme and vice versa.

The main types of regulators used in control systems for electric drives of drilling rig actuators

Analog controllers in systems of slave control of electric drives are built on the basis of operational amplifiers (op-amps) - DC amplifiers with high input and very low output impedances. The technology of integrated circuits makes it possible at present to manufacture high-quality and inexpensive op amps. In some part of its operating range, the op amp behaves like a linear voltage amplifier with a very high gain (10 5 - 10 6). If the op-amp circuit does not provide for negative feedback from the output to the input, then due to the high gain, it will necessarily enter the saturation mode. Therefore, op-amp-based controller circuits contain negative feedback.
The op amp gets its name from the fact that it can perform various mathematical operations such as multiplication, summation, integration, and differentiation. Typical regulators are built on the basis of an inverting amplifier, and the input and output circuits, in addition to resistances, may contain capacitances.
Since the gain of the op amp is high (Ku= = 10 5 +10 6), and the output voltage Uout is limited by the supply voltage CPU, then the potential of the point BUT(Fig. 1, a) cpA = wout/Ku is close to zero, i.e. dot BUT performs the function of an apparent earth (to ground a point BUT not possible, otherwise the circuit will become inoperable).

Rice. Fig. 1. The structure of an analog regulator based on an operational amplifier (a). Scheme of a proportional regulator with a controlled limitation of the output signal (b). Input-output characteristic of the controller with controlled output signal limitation (c)

Schemes, transfer functions and transition functions of regulators of various types are given in Table.

Schemes and dynamic characteristics of various types of regulators



To obtain a proportional controller (P-regulator), resistors are included at the input and in the feedback circuit of the OS; an integral regulator (I-regulator) includes a resistor in the input circuit, and a capacitor in the feedback circuit; PI controller into the input circuit-resistor, and into the feedback circuit - series-connected resistor and capacitor. The PID controller can be implemented on a single amplifier using active-capacitive circuits at the input and in the feedback circuit.
The industry produces various types of operational amplifiers on integrated circuits (ICs) - both round and rectangular. The most widespread for the construction of regulators were op-amps of types K140UD7, K553UD2, K157UD2, etc.
It is possible to reduce the size and increase the reliability of devices of analog electric drive control systems by introducing a hybrid technology for their manufacture. In the manufacture of hybrid integrated circuits (HIC), active elements (OC) are installed on a printed circuit board in a solid-state (unpackaged) design, and capacitors and resistors are installed using film technology (sputtering films from conductive, semi-conductive and non-conductive materials). The resulting module can be filled with a compound or placed in a housing.
Limitation of the coordinates of the electric drive (current, speed, etc.) is carried out by including the limiting nodes in the controller structure of the external control loop. The latter can be managed and unmanaged. In fig., 6 shows a diagram of limiting the output voltage of a proportional regulator with cut-off diodes VD1, VD2 and a controlled reference voltage Vop. The circuit allows you to get an input-output characteristic that is asymmetric with respect to the origin with a different level of limited output voltage (Fig.) There are other options for controlled limiting of the output voltage of the op-amp using transistors.
Until recently, in the automated electric drive of the actuators of domestic drilling rigs, the main use was made of analog computer technology. In recent years, a number of design and research organizations have been working on the creation of microprocessor control systems. Compared with analog systems, microprocessor systems have a number of advantages. Let's note some of them.
Flexibility. Possibility by reprogramming to change not only the parameters of the control system, but also the algorithms and even the structure. At the same time, the hardware of the system remains unchanged. In analog systems, a hardware re-arrangement would be required. The microcomputer software can be easily adjusted both during the pre-launch period and during their operation. Due to this, the costs and terms of adjustment work are reduced and their nature changes, since the necessary experiments to determine the characteristics and parameters, as well as adjusting the controllers, can be performed automatically by the microcomputer itself according to a pre-prepared program.
Removal of all restrictions on the structure of the control device and control laws. At the same time, the quality indicators of digital systems can significantly exceed the quality indicators of control of continuous control systems. By introducing appropriate programs, complex control laws (optimization, adaptation, forecasting, etc.) can be implemented, including those that are very difficult to implement using analog tools. It becomes possible to solve intellectual problems that ensure the correctness and efficiency of conducting technological processes. On the basis of a microcomputer, systems of any type can be built, including systems with subordinate control, multidimensional systems with cross-connections, etc.
Self-diagnosis and self-test digital control devices. Possibility to check the serviceability of mechanical drive units, power converters, sensors and other equipment during technological pauses, i.e. automatic diagnostics of equipment condition and early warning of accidents. These capabilities are complemented by advanced anti-jamming tools. The main thing here is the replacement of analog data transmission lines with digital ones containing galvanic isolation, fiber optic channels, noise-immune integrated circuits as amplifiers and switches.
Higher Accuracy due to the absence of zero drift characteristic of analog devices. Thus, digital systems for controlling the speed of an electric drive can provide an increase in the accuracy of regulation by two orders of magnitude compared to analog ones.
Ease of Visualization parameters of the control process by using digital indicators, indicator panels and displays, organizing an interactive mode of information exchange with the operator.
Greater reliability, smaller dimensions, weight and cost. The high reliability of microcomputers in comparison with analog technology is ensured by the use of large integrated circuits (LSI), the presence of special memory protection systems, noise immunity and other means. Due to the high level of LSI production technology, the cost of manufacturing control systems for electric drives is reduced. These advantages are especially evident when using single-board and single-chip computers.

The purpose of the regulators is to set and maintain at a given level (setting parameter) a certain physical value X (adjustable value). To do this, the regulator must counteract the effects of disturbances in a certain way.

A schematic block diagram of a simple control loop is shown in fig. 26.1. The controller influences the controlled variable X by means of a control variable in such a way that the control deviation is as small as possible. The perturbation affecting the control object can be formally represented by the magnitude of the interference additively superimposed on the setting parameter. Below we will proceed from the assumption that the controlled variable is an electric voltage and that the object is adjusted electrically. Therefore, an electronic controller can be used.

The simplest example of such a controller is an amplifier, the input of which is the deviation of the controlled value. If the controlled value X exceeds the specified value, the difference becomes negative. As a result, the control action Y is reduced on a correspondingly larger scale. This reduction compensates for the difference. In the steady state, the residual mismatch is the smaller, the higher the controller gain. For the linear system presented in fig. 26.1, the relations are valid

Rice. 26.1. Block diagram of the control loop.

From here we obtain an expression for determining the controlled variable

It is clear that the ability of the system to follow a change in the setting parameter is closer to 1, the higher the gain of the feedback loop:

The transient response when disturbed is the closer to zero, the greater the gain of the controller. However, one should take into account the fact that the gain of the feedback loop cannot be made arbitrarily large, since then the inevitable phase shift in the control loop will lead to oscillations. We have already encountered a similar problem when considering the correction of the frequency response of operational amplifiers. The task of regulation is to ensure, despite these limitations, the smallest possible mismatch of regulation and a good transient response. To this end, an integrator and a differentiator are added to the linear amplifier, and in this way, instead of a proportional controller (controller), a PI or PID controller is obtained. The following sections are devoted to the implementation of such a controller using electronic circuits.

The journey of ten thousand miles begins with the first step.
(Chinese proverb)

It was in the evening, there was nothing to do ... And so suddenly I wanted to solder something. Sort of ... Electronic! .. Solder - so solder. The computer is available, the Internet is connected. We choose a scheme. And suddenly it turns out that the schemes for the conceived subject are a wagon and a small cart. And everyone is different. No experience, little knowledge. Which one to choose? Some of them contain some kind of rectangles, triangles. Amplifiers, and even operational ones ... How they work is not clear. Stra-a-ashno! .. What if it burns down? We choose what is simpler, on familiar transistors! Chose, soldered, turned on ... HELP !!! Does not work!!! Why?

Yes, because "Simplicity is worse than theft"! It's like a computer: the fastest and most sophisticated - gaming! And for office work, the simplest is enough. It's the same with transistors. Soldering a circuit on them is not enough. You still need to know how to set it up. Too many "pitfalls" and "rake". And this often requires experience that is by no means an entry level. So what, quit an exciting activity? By no means! Just do not be afraid of these "triangles-rectangles". It turns out that in many cases it is much easier to work with them than with individual transistors. IF YOU KNOW - HOW!

Here we are: understanding how an operational amplifier (op-amp, or in English OpAmp) works, we will now deal with. At the same time, we will consider his work literally “on the fingers”, practically without using any formulas, except perhaps, except for Ohm’s grandfather’s law: “Current through a circuit section ( I) is directly proportional to the voltage across it ( U) and inversely proportional to its resistance ( R)»:
I=U/R. (1)

To begin with, in principle, it is not so important how exactly the op-amp is arranged inside. Let's just take as an assumption that it is a "black box" with some stuffing there. On the this stage we will not consider such parameters of the op-amp as “bias voltage”, “shift voltage”, “temperature drift”, “noise characteristics”, “common-mode suppression coefficient”, “supply voltage ripple suppression coefficient”, “bandwidth”, etc. .P. All these parameters will be important at the next stage of its study, when the basic principles of its work “settle down” in the head, because “it was smooth on paper, but forgot about the ravines” ...

For now, let's just assume that the parameters of the op-amp are close to ideal and consider only what signal will be at its output if some signals are applied to its inputs.

So, the operational amplifier (op-amp) is a DC differential amplifier with two inputs (inverting and non-inverting) and one output. In addition to them, the op-amp has power leads: positive and negative. These five conclusions are found in nearly any OS and are fundamentally necessary for its operation.

The op-amp has a huge gain, at least 50,000 ... 100,000, but in reality - much more. Therefore, as a first approximation, we can even assume that it is equal to infinity.

The term "differential" ("different" is translated from English as "difference", "difference", "difference") means that the output potential of the op-amp is affected exclusively by the potential difference between its inputs, regardless from them absolute meaning and polarity.

The term "DC" means that the op-amp amplifies the input signals starting from 0 Hz. The upper frequency range (frequency range) of the signals amplified by the op amp depends on many factors, such as frequency characteristics transistors of which it consists, the gain of a circuit built using an op-amp, etc. But this issue is already beyond the scope of the initial acquaintance with his work and will not be considered here.

Op-amp inputs have a very high input impedance equal to tens/hundreds of MegaOhm, or even GigaOhm (and only in the memorable K140UD1, and even in K140UD5 it was only 30...50 kOhm). Such a high impedance of the inputs means that they have almost no effect on the input signal.

Therefore, with a high degree of approximation to the theoretical ideal, we can assume that current does not flow into the inputs of the op-amp . This - first an important rule that is applied in the analysis of the operation of the OS. Please remember well what it concerns only the OU itself, but not schemes with its use!

What do the terms "inverting" and "non-inverting" mean? In relation to what is the inversion determined and, in general, what kind of “animal” is this - signal inversion?

Translated from Latin, one of the meanings of the word "inversio" is "wrapping", "coup". In other words, inversion is a mirror image ( mirroring) signal relative to the horizontal axis X(time axis). On Fig. 1 shows a few of the many possible signal inversion options, where the direct (input) signal is marked in red and the inverted (output) signal is in blue.

Rice. 1 Concept of signal inversion

It should be especially noted that to the zero line (as in Fig. 1, A, B) the signal inversion not tied! Signals can be inverse and asymmetrical. For example, both are only in the region of positive values ​​(Fig. 1, B), which is typical for digital signals or with unipolar power supply (this will be discussed later), or both are partially in the positive and partially in the negative regions (Fig. 1, B, D). Other options are also possible. The main condition is their mutual specularity relative to some arbitrarily chosen level (for example, an artificial midpoint, which will also be discussed later). In other words, polarity signal is also not a determining factor.

Depict OU on circuit diagrams in different ways. Abroad, OS were previously depicted, and even now they are very often depicted in the form of an isosceles triangle (Fig. 2, A). The inverting input is marked with a minus symbol, and the non-inverting input is marked with a plus symbol inside a triangle. These symbols do not mean at all that the potential at the respective inputs must be more positive or more negative than at the other. They simply indicate how the output potential reacts to the potentials applied to the inputs. As a result, they are easy to confuse with power leads, which can be an unexpected "rake", especially for beginners.


Rice. 2 Variants of conditional graphic images (UGO)
operational amplifiers

In the system of domestic conditional graphic images (UGO) before the entry into force of GOST 2.759-82 (ST SEV 3336-81), OUs were also depicted as a triangle, only the inverting input - with an inversion symbol - a circle at the intersection of the output with a triangle (Fig. 2, B), and now - in the form of a rectangle (Fig. 2, C).

When designating the op-amp on the diagrams, the inverting and non-inverting inputs can be interchanged if it is more convenient, however, traditionally, the inverting input is shown at the top, and the non-inverting input is at the bottom. Power pins are usually always placed in one way (positive at the top, negative at the bottom).

Op-amps are almost always used in negative feedback (NFB) circuits.

Feedback is the effect of applying a portion of the output voltage of an amplifier to its input, where it is algebraically (subject to sign) added to the input voltage. The principle of signal summation will be discussed below. Depending on which input of the op-amp, inverting or non-inverting, the OS is fed, there is a negative feedback (NFB), when part of the output signal is applied to the inverting input (Fig. 3, A) or positive feedback (PIC), when a part the output signal is fed, respectively, to the non-inverting input (Fig. 3, B).


Rice. 3 The principle of feedback formation (OS)

In the first case, since the output is the inverse of the input, it is subtracted from the input. As a result, the overall gain of the stage is reduced. In the second case, it is added to the input, the overall gain of the cascade is increased.

At first glance, it may seem that POS has a positive effect, and OOS is a completely useless undertaking: why reduce the gain? This is exactly what U.S. patent examiners thought when, in 1928, Harold S. Black tried patent the OS. However, sacrificing amplification, we significantly improve other important parameters of the circuit, such as its linearity, frequency range, etc. The deeper the OOS, the less the characteristics of the entire circuit depend on the characteristics of the op-amp.

But the PIC (given its own huge gain of the op-amp) has the opposite effect on the characteristics of the circuit and the most unpleasant thing is that it causes its self-excitation. Of course, it is also used consciously, for example, in generators, comparators with hysteresis (more on this later), etc., but in general, its effect on the operation of amplifier circuits with an op-amp is rather negative and requires a very thorough and reasonable analysis its application.

Since the OS has two inputs, the following main types of its inclusion using the OS are possible (Fig. 4):


Rice. 4 Basic schemes for switching on the OS

but) inverting (Fig. 4, A) - the signal is applied to the inverting input, and the non-inverting one is connected directly to the reference potential (not used);

b) non-inverting (Fig. 4, B) - the signal is applied to the non-inverting input, and the inverting one is connected directly to the reference potential (not used);

in) differential (Fig. 4, B) - signals are fed to both inputs, inverting and non-inverting.

To analyze the operation of these schemes, one should take into account second the most important rule, to which the operation of the OS is subject: The output of an op-amp tends to have zero voltage difference between its inputs..

However, any wording must be necessary and sufficient to limit the entire subset of cases that obey it. The above formulation, for all its “classicism”, does not give any information about which of the inputs the output “seeks to influence”. Based on it, it turns out that the op-amp seems to equalize the voltages at its inputs, applying voltage to them from somewhere “from the inside”.

Looking closely at the diagrams in Fig. 4, you can see that the OOC (through Rooc) in all cases is started from the exit only to the inverting input, which gives us reason to reformulate this rule as follows: Voltage on the output of the op-amp, covered by the OOS, tends to ensure that the potential at the inverting input is equal to the potential at the non-inverting input.

Based on this definition, the “leading” at any inclusion of the OA with OOS is the non-inverting input, and the “slave” is the inverting one.

When describing the operation of an op amp, the potential at its inverting input is often referred to as "virtual zero" or "virtual midpoint". The translation of the Latin word "virtus" means "imaginary", "imaginary". A virtual object behaves close to the behavior of similar objects of material reality, i.e., for input signals (due to the action of the FOS), the inverting input can be considered connected directly to the same potential as the non-inverting input. However, "virtual zero" is just a special case that takes place only with bipolar power supply of the op-amp. When using a unipolar power supply (which will be discussed below), and in many other switching circuits, there will be no zero on either the non-inverting or inverting inputs. Therefore, let's agree that we will not use this term, since it interferes with the initial understanding of the principles of operation of the OS.

From this point of view, we will analyze the schemes shown in Fig. 4. At the same time, to simplify the analysis, we will assume that the supply voltages are still bipolar, equal to each other in value (say, ± 15 V), with a midpoint (common bus or “ground”), relative to which we will count the input and output voltages. In addition, the analysis will be carried out in direct current, because. a changing alternating signal at each moment of time can also be represented as a sample of direct current values. In all cases, feedback through Rooc is connected from the output of the op-amp to its inverting input. The difference is only in which of the inputs the input voltage is applied.

BUT) inverting switching on (Fig. 5).


Rice. 5 The principle of operation of the op-amp in an inverting connection

The potential at the non-inverting input is zero, because it is connected to the midpoint ("ground"). An input signal equal to +1 V relative to the midpoint (from GB) is applied to the left terminal of the input resistor Rin. Let us assume that the resistances Rooc and Rin are equal to each other and amount to 1 kOhm (their total resistance is 2 kOhm).

According to Rule 2, the inverting input must have the same potential as the non-inverting input, i.e., 0 V. Therefore, a voltage of +1 V is applied to Rin. According to Ohm's law, a current will flow through it Iinput= 1 V / 1000 ohms = 0.001 A (1 mA). The direction of flow of this current is shown by an arrow.

Since Rooc and Rin are connected by a divider, and according to Rule 1, the op-amp inputs do not consume current, in order for the voltage to be 0 V at the midpoint of this divider, a voltage must be applied to the right output of Rooc minus 1 V, and the current flowing through it Ioos should also be equal to 1 mA. In other words, a voltage of 2 V is applied between the left terminal Rin and the right terminal Rooc, and the current flowing through this divider is 1 mA (2 V / (1 kΩ + 1 kΩ) = 1 mA), i.e. I input = I oos .

If a negative polarity voltage is applied to the input, the output of the op-amp will be a positive polarity voltage. Everything is the same, only the arrows showing the flow of current through Rooc and Rin will be directed in the opposite direction.

Thus, if the values ​​​​of Rooc and Rin are equal, the voltage at the output of the op-amp will be equal to the voltage at its input in magnitude, but inverse in polarity. And we got inverting repeater . This scheme is often used if you need to invert the signal received using circuits that are fundamentally inverters. For example, logarithmic amplifiers.

Now let's keep Rin equal to 1 kOhm and increase the resistance Rooc to 2 kOhm with the same input signal +1 V. The total divider resistance Rooc+Rin has increased to 3 kOhm. In order for a potential of 0 V (equal to the potential of the non-inverting input) to remain at its midpoint, the same current (1 mA) must flow through Rooc as through Rin. Therefore, the voltage drop across Rooc (voltage at the output of the op-amp) should already be 2 V. At the output of the op-amp, the voltage is minus 2 V.

Let's increase the value of Rooc to 10 kOhm. Now the voltage at the output of the op-amp under the same other conditions will already be 10 V. Wow! Finally we got inverting amplifier ! Its output voltage is greater than the input voltage (in other words, the gain Ku) as many times as the resistance Rooc is greater than the resistance Rin. No matter how I swore not to use formulas, let's still display this as an equation:
Ku \u003d - Uout / Uin \u003d - Rooc / Rin. (2)

The minus sign in front of the fraction on the right side of the equation only means that the output signal is inverse with respect to the input. And nothing more!

And now let's increase the resistance Rooc to 20 kOhm and analyze what happens. According to formula (2), with Ku \u003d 20 and an input signal of 1 V, the output should have been a voltage of 20 V. But it wasn’t there! We previously assumed that the supply voltage of our op-amp is only ± 15 V. But even 15 V cannot be obtained (why so - a little lower). "You can't jump above your head (supply voltage)"! As a result of such abuse of the ratings of the circuit, the output voltage of the op-amp “rests” on the supply voltage (the output of the op-amp enters saturation). The balance of current equality through the divider RoocRin ( Iinput = Ioos) is violated, a potential appears at the inverting input, which is different from the potential at the non-inverting input. Rule 2 no longer applies.

Input resistance inverting amplifier is equal to the resistance Rin, since all the current from the input signal source (GB) flows through it.

Now let's replace the constant Rooc with a variable, with a nominal value of, say, 10 kOhm (Fig. 6).


Rice. 6 Variable gain inverting amplifier circuit

With the right (according to the circuit) position of its slider, the gain will be Rooc / Rin \u003d 10 kOhm / 1 kOhm = 10. By moving the Rooc slider to the left (decreasing its resistance), the gain of the circuit will decrease and, finally, at its extreme left position it will become equal to zero, since the numerator in the above formula will become zero at any the value of the denominator. The output will also be zero for any value and polarity of the input signal. Such a scheme is often used in audio signal amplification circuits, for example, in mixers, where you have to adjust the gain from zero.

B) non-inverting switching on (Fig. 7).


Rice. 7 The principle of operation of the op-amp in a non-inverting inclusion

The left pin of Rin is connected to the midpoint ("ground"), and the input signal equal to +1 V is applied directly to the non-inverting input. Since the nuances of the analysis are “chewed” above, here we will pay attention only to significant differences.

At the first stage of the analysis, we also take the resistances Rooc and Rin equal to each other and equal to 1 kOhm. Because at the non-inverting input, the potential is +1 V, then according to Rule 2, the same potential (+1 V) must be at the inverting input (shown in the figure). To do this, there must be a voltage of +2 V on the right terminal of the Rooc resistor (output of the op-amp). Currents Iinput And Ioos, equal to 1 mA, now flow through the resistors Rooc and Rin in the opposite direction (shown by arrows). We got it non-inverting amplifier with a gain of 2, since an input of +1V produces an output of +2V.

Strange, isn't it? The ratings are the same as in the inverting connection (the only difference is that the signal is applied to another input), and the gain is obvious. We'll look into this a little later.

Now we increase the value of Rooc to 2 kOhm. To keep the balance of currents Iinput = Ioos and the potential of the inverting input is +1 V, the output of the op-amp should already be +3 V. Ku \u003d 3 V / 1 V \u003d 3!

If we compare the values ​​of Ku with a non-inverting connection with an inverting one, with the same ratings Rooc and Rin, it turns out that the gain in all cases is greater by one. We derive the formula:
Ku \u003d Uout / Uin + 1 \u003d (Rooc / Rin) + 1 (3)

Why is this happening? Yes, very easy! The NFB works exactly the same as in an inverting connection, but according to Rule 2, the potential of the non-inverting input is always added to the potential of the inverting input in a non-inverting connection.

So, with a non-inverting inclusion, it is impossible to obtain a gain equal to 1? Why not, why not. Let's reduce the value of Rooc, similar to how we analyzed Fig. 6. With its zero value - by short-circuiting the output with the inverting input (Fig. 8, A), according to Rule 2, the output will have such a voltage that the potential of the inverting input is equal to the potential of the non-inverting input, i.e., +1 V. We get: Ku \u003d 1 V / 1 V \u003d 1 (!) Well, since the inverting input does not consume current and there is no potential difference between it and the output, then no current flows in this circuit.


Rice. 8 Scheme of switching on the op-amp as a voltage follower

Rin becomes generally superfluous, because it is connected in parallel with the load on which the output of the op-amp should work, and its output current will flow through it in vain. And what happens if you leave Rooc, but remove Rin (Fig. 8, B)? Then in the gain formula Ku = Roos / Rin + 1, the resistance Rin theoretically becomes close to infinity (in reality, of course, not, because there are leaks on the board, and the input current of the op-amp, although negligible, is still zero is still not equal), and the ratio Rooc / Rin is equated to zero. Only one remains in the formula: Ku \u003d + 1. Can the gain be less than one for this circuit? No, less will not work under any circumstances. You can’t go around the “extra” unit in the gain formula on a crooked goat ...

After we removed all the "extra" resistors, we get a circuit non-inverting repeater shown in Fig. 8, V.

At first glance, such a scheme does not make practical sense: why do we need a single, and even non-inverse "amplification" - what, you can’t just send a signal further ??? However, such schemes are used quite often and here's why. According to Rule 1, current does not flow into the inputs of the op-amp, i.e., input impedance the non-inverting follower is very large - the same tens, hundreds and even thousands of MΩ (the same applies to the circuit according to Fig. 7)! But the output resistance is very small (fractions of Ohm!). The output of the op-amp “chuffs with all its might”, trying, according to Rule 2, to maintain the same potential at the inverting input as at the non-inverting one. The only limitation is the permissible output current of the op-amp.

But from this place we will wag a little to the side and consider the issue of the output currents of the op-amp in a little more detail.

For most general purpose op amps, the technical specifications state that the resistance of the load connected to their output should not be less 2 kOhm More - as much as you want. For a much smaller number, it is 1 kOhm (K140UD ...). This means that under the worst-case conditions: the maximum supply voltage (e.g. ±16 V or a total of 32 V), a load connected between the output and one of the power rails, and the maximum output voltage of opposite polarity, a voltage of about 30 V will be applied to the load. In this case, the current through it will be: 30 V / 2000 Ohm = 0.015 A (15 mA). Not so little, but not too much either. Fortunately, most general purpose op amps have built-in overcurrent protection - typical maximum output current is 25 mA. Protection prevents overheating and failure of the op-amp.

If the supply voltages are not the maximum allowable, then the minimum load resistance can be proportionally reduced. Say, with a power supply of 7.5 ... 8 V (total 15 ... 16 V), it can be 1 kOhm.

IN) differential switching on (Fig. 9).


Rice. 9 The principle of operation of the op-amp in a differential connection

So, let's assume that with the same ratings Rin and Rooc equal to 1 kOhm, the same voltages equal to +1 V are applied to both inputs of the circuit (Fig. 9, A). Since the potentials on both sides of the resistor Rin are equal to each other (the voltage across the resistor is 0), no current flows through it. This means that the current through the resistor Rooc is also zero. That is, these two resistors do not perform any function. In fact, we actually got a non-inverting follower (compare with Fig. 8). Accordingly, we will get the same voltage at the output as at the non-inverting input, i.e., +1 V. Let's change the polarity of the input signal at the inverting input of the circuit (turn GB1 over) and apply minus 1 V (Fig. 9, B). Now a voltage of 2 V is applied between the terminals Rin and a current flows through it Iin\u003d 2 mA (I hope that it is no longer necessary to describe in detail why this is so?). In order to compensate for this current, a current of 2 mA must also flow through Rooc. And for this, the output of the op-amp must have a voltage of +3 V.

That's where the malicious "grin" of an additional one appeared in the formula for the gain of a non-inverting amplifier. It turns out that with such simplified In differential switching, the difference in gain constantly shifts the output signal by the potential at the non-inverting input. A problem with! However, "Even if you were eaten, you still have at least two exits." This means that we somehow need to equalize the gains of the inverting and non-inverting inclusions in order to “neutralize” this extra one.

To do this, let's apply the input signal to the non-inverting input not directly, but through the divider Rin2, R1 (Fig. 9, B). Let's take their denominations also for 1 kOhm. Now, at the non-inverting (and therefore also at the inverting) input of the op-amp, there will be a potential of +0.5 V, a current will flow through it (and Rooc) Iin = Ioos\u003d 0.5 mA, to ensure which the output of the op-amp must have a voltage equal to 0 V. Phew! We got what we wanted! With equal magnitude and polarity signals at both inputs of the circuit (in this case +1 V, but the same will be true for minus 1 V and for any other digital values), the output of the op-amp will maintain zero voltage equal to the difference in input signals .

Let's check this reasoning by applying a signal of negative polarity minus 1 V to the inverting input (Fig. 9, D). Wherein Iin = Ioos= 2 mA, for which the output should be +2 V. Everything was confirmed! The output level corresponds to the difference between the inputs.

Of course, if Rin1 and Rooc are equal (respectively, Rin2 and R1), we will not get amplification. To do this, you need to increase the values ​​​​of Rooc and R1, as was done when analyzing previous inclusions of the op-amp (I will not repeat it), and it should strictly respect the ratio:

Rooc / Rin1 = R1 / Rin2. (4)

What useful do we get from such an inclusion in practice? And we get a remarkable property: the output voltage does not depend on the absolute values ​​of the input signals, if they are equal to each other in magnitude and polarity. Only the difference (differential) signal is output. This makes it possible to amplify very small signals against the background of noise acting equally on both inputs. For example, a signal from a dynamic microphone against the background of a 50 Hz industrial frequency mains pickup.

However, in this barrel of honey, unfortunately, there is a fly in the ointment. First, equality (4) must be observed very strictly (up to tenths and sometimes hundredths of a percent!). Otherwise, there will be an unbalance of the currents acting in the circuit, and therefore, in addition to the difference ("anti-phase") signals, the combined ("common-mode") signals will also be amplified.

Let's understand the essence of these terms (Fig. 10).


Rice. 10 Signal phase shift

The phase of the signal is a value that characterizes the offset of the origin of the signal period relative to the origin of time. Since both the origin of time and the origin of the period are chosen arbitrarily, the phase of one periodical signal has no physical meaning. However, the phase difference between the two periodical signals is a quantity that has a physical meaning, it reflects the delay of one of the signals relative to the other. What is considered the beginning of the period does not matter. For the point of the beginning of the period, you can take a zero value with a positive slope. It is possible - maximum. Everything is in our power.

On Fig. 9, red indicates the original signal, green - shifted by ¼ period relative to the original, and blue - by ½ period. If we compare the red and blue curves with the curves in Fig. 2, B, it can be seen that they are mutually inverse. Thus, “in-phase signals” are signals that coincide with each other at each of their points, and “anti-phase signals” are inverse relative to each other.

At the same time, the concept inversions broader than the concept phases, because the latter applies only to regularly repeated, periodic signals. And the concept inversions applicable to any signals, including non-periodic ones, such as an audio signal, a digital sequence, or a constant voltage. To phase is a consistent value, the signal must be periodic at least over a certain interval. Otherwise, both phase and period turn into mathematical abstractions.

Secondly, the inverting and non-inverting inputs in the differential connection, with equal ratings Rooc = R1 and Rin1 = Rin2, will have different input resistances. If the input resistance of the inverting input is determined only by the value Rin1, then the non-inverting input is determined by the values successively included Rin2 and R1 (have not forgotten that the op-amp inputs do not consume current?). In the example above, they will be 1 and 2 kΩ, respectively. And if we increase Rooc and R1 to obtain a full-fledged amplifying stage, then the difference will increase even more significantly: with Ku \u003d 10 - up to, respectively, all the same 1 kOhm and as much as 11 kOhm!

Unfortunately, in practice, the ratings Rin1 = Rin2 and Rooc = R1 are usually set. However, this is only acceptable if the signal sources for both inputs are of very low output impedance. Otherwise, it forms a divider with the input impedance of this amplifying stage, and since the division factor of such “dividers” will be different, the result is obvious: a differential amplifier with such resistor values ​​will not perform its function of suppressing common-mode (combined) signals, or perform this function poorly .

One of the ways to solve this problem can be the inequality of the values ​​of the resistors connected to the inverting and non-inverting inputs of the op-amp. Namely, so that Rin2 + R1 = Rin1. Another important point is to achieve exact observance of equality (4). As a rule, this is achieved by splitting R1 into two resistors - a constant, usually 90% of the desired value, and a variable (R2), whose resistance is 20% of the required value (Fig. 11, A).


Rice. 11 Differential amplifier balancing options

The path is generally accepted, but again, with this method of balancing, albeit slightly, the input impedance of the non-inverting input changes. A much more stable option with the inclusion of a tuning resistor (R5) in series with Rooc (Fig. 11, B), since Rooc does not participate in the formation of the input resistance of the inverting input. The main thing is to keep the ratio of their denominations, similar to option "A" (Rooc / Rin1 = R1 / Rin2).

Since we talked about differential switching and mentioned repeaters, I would like to describe one interesting circuit (Fig. 12).


Rice. 12 Switched inverting/non-inverting follower circuit

The input signal is applied simultaneously to both inputs of the circuit (inverting and non-inverting). The ratings of all resistors (Rin1, Rin2 and Rooc) are equal to each other (in this case, let's take their real values: 10 ... 100 kOhm). The non-inverting input of the op-amp with the SA key can be closed to a common bus.

In the closed position of the key (Fig. 12, A), the resistor Rin2 does not participate in the operation of the circuit (only current “uselessly” flows through it Ivx2 from the signal source to the common bus). We get inverting follower with a gain equal to minus 1 (see Fig. 6). But with the SA key in the open position (Fig. 12, B), we get non-inverting follower with gain equal to +1.

The principle of operation of this scheme can be expressed in a slightly different way. When the SA key is closed, it works as an inverting amplifier with a gain equal to minus 1, and when it is open - simultaneously(!) And as an inverting amplifier with a gain, minus 1, and as a non-inverting amplifier with a gain of +2, from where: Ku = +2 + (–1) = +1.

In this form, this circuit can be used if, for example, the polarity of the input signal is unknown at the design stage (say, from a sensor that is not accessible until the device is set up). If, however, a transistor (for example, a field-effect transistor) is used as a key, controlled from the input signal using comparator(which will be discussed below), we get synchronous detector(synchronous rectifier). The specific implementation of such a scheme, of course, goes beyond the initial acquaintance with the operation of the OS, and we will not consider it in detail here again.

And now let's consider the principle of summing the input signals (Fig. 13, A), and at the same time we will figure out what values ​​​​of the resistors Rin and Rooc should be in reality.


Rice. 13 The principle of operation of the inverting adder

We take as a basis the inverting amplifier already discussed above (Fig. 5), only we connect not one, but two input resistors Rin1 and Rin2 to the input of the op-amp. So far, for "educational" purposes, we accept the resistance of all resistors, including Rooc, equal to 1 kOhm. We supply input signals equal to +1 V to the left terminals Rin1 and Rin2. Currents equal to 1 mA flow through these resistors (shown by arrows pointing from left to right). To maintain the same potential at the inverting input as at the non-inverting one (0 V), a current equal to the sum of the input currents (1 mA + 1 mA = 2 mA) must flow through the Rooc resistor, shown by an arrow pointing in the opposite direction (from right to left ), for which the output of the op-amp must have a voltage of minus 2 V.

The same result (output voltage minus 2 V) can be obtained if +2 V is applied to the input of the inverting amplifier (Fig. 5), or the value of Rin is halved, i.e. up to 500 Ohm. Let's increase the voltage applied to the resistor Rin2 up to +2 V (Fig. 13, B). At the output we get a voltage of minus 3 V, which is equal to the sum of the input voltages.

There can be not two inputs, but as many as you like. The principle of operation of this circuit will not change from this: the output voltage in any case will be directly proportional to the algebraic sum (taking into account the sign!) of the currents passing through the resistors connected to the inverting input of the op-amp (inversely proportional to their ratings), regardless of their number.

If, on the other hand, signals equal to +1 V and minus 1 V are applied to the inputs of the inverting adder (Fig. 13, B), then the currents flowing through them will be in different directions, they will cancel each other out and the output will be 0 V. Through the resistor Rooc in this case no current will flow. In other words, the current flowing through Rooc is algebraically summed with input currents.

An important point also follows from this: while we were operating with small input voltages (1 ... 3 V), the output of a widely used op-amp could well provide such a current (1 ... 3 mA) for Rooc and something else remained for the load connected to the output of the op-amp. But if the voltages of the input signals are increased to the maximum allowable (close to the supply voltages), then it turns out that the entire output current will go to Rooc. Nothing left to load. And who needs an amplifying stage that works "for itself"? In addition, input resistor values ​​of only 1 kΩ (respectively, determining the input resistance of the inverting amplifier stage) require excessively high currents to flow through them, heavily loading the signal source. Therefore, in real circuits, the resistance Rin is chosen not less than 10 kOhm, but it is also desirable not more than 100 kOhm, so that at a given gain, Rooc is not set too high. Although these values ​​\u200b\u200bare not absolute, but only estimates, as they say, "in the first approximation" - it all depends on the specific circuit. In any case, it is undesirable that a current flowing through Rooc exceeds 5 ... 10% of the maximum output current of this particular op-amp.

The summed signals can also be applied to the non-inverting input. It turns out non-inverting adder. In principle, such a circuit will work in exactly the same way as an inverting adder, the output of which will be a signal that is directly proportional to the input voltages and inversely proportional to the values ​​of the input resistors. However, in practice it is used much less frequently, because. contains a "rake" that should be taken into account.

Since Rule 2 is valid only for the inverting input, which has a “virtual zero potential”, then the non-inverting input will have a potential equal to the algebraic sum of the input voltages. Therefore, the input voltage available at one of the inputs will affect the voltage supplied to the other inputs. There is no “virtual potential” at the non-inverting input! As a result, additional circuitry tricks have to be applied.

So far, we have considered circuits based on OS with OOS. What happens if feedback is removed altogether? In this case, we get comparator(Fig. 14), i.e., a device that compares the absolute value of two potentials at its inputs (from the English word compare- compare). At its output, there will be a voltage approaching one of the supply voltages, depending on which of the signals is greater than the other. Typically, the input signal is applied to one of the inputs, and to the other - a constant voltage with which it is compared (the so-called "reference voltage"). It can be anything, including zero potential (Fig. 14, B).


Rice. 14 Scheme of switching on the op-amp as a comparator

However, not everything is so good "in the kingdom of Denmark" ... And what happens if the voltage between the inputs is zero? In theory, the output should also be zero, but in reality - never. If the potential at one of the inputs even slightly outweighs the potential of the other, then this will already be enough for chaotic voltage surges to occur at the output due to random disturbances induced at the inputs of the comparator.

In reality, any signal is "noisy", because ideal cannot be by definition. And in the area close to the point of equality of the potentials of the inputs, a burst of output signals will appear at the output of the comparator instead of one clear switching. To combat this phenomenon, the comparator circuit is often introduced hysteresis by creating a weak positive PIC from the output to the non-inverting input (Figure 15).


Rice. 15 The principle of operation of the hysteresis in the comparator due to the POS

Let's analyze the operation of this scheme. Its supply voltage is ± 10 V (for an even account). The resistance Rin is 1 kOhm, and Rpos is 10 kOhm. The midpoint potential is chosen as the reference voltage applied to the inverting input. The red curve shows the input signal coming to the left pin Rin (input schemes comparator), blue - the potential at the non-inverting input of the op-amp and green - the output signal.

While the input signal has a negative polarity, the output is a negative voltage, which, through Rpos, is added to the input voltage in inverse proportion to the values ​​of the corresponding resistors. As a result, the potential of the non-inverting input in the entire range of negative values ​​is 1 V (in absolute value) higher than the input signal level. As soon as the potential of the non-inverting input is equal to the potential of the inverting one (for the input signal, this will be + 1 V), the voltage at the output of the op-amp will begin to switch from negative to positive polarity. The total potential at the non-inverting input will start like an avalanche become even more positive, supporting the process of such a switch. As a result, the comparator simply “will not notice” insignificant noise fluctuations of the input and reference signals, since they will be many orders of magnitude smaller in amplitude than the described “step” of the potential at the non-inverting input when switching.

With a decrease in the input signal, the reverse switching of the output signal of the comparator will occur at an input voltage of minus 1 V. This difference between the input signal levels leading to the switching of the output of the comparator, equal in our case to a total of 2 V, is called hysteresis. The greater the resistance Rpos with respect to Rin (the smaller the POS depth), the smaller the switching hysteresis. So, with Rpos \u003d 100 kOhm, it will be only 0.2 V, and with Rpos \u003d 1 MΩ, it will be 0.02 V (20 mV). The hysteresis (PIC depth) is selected based on the actual operating conditions of the comparator in a particular circuit. In which 10 mV will be a lot, and in which - and 2 V will be small.

Unfortunately, not every op amp and not in all cases can be used as a comparator. Specialized comparator microcircuits are produced for matching between analog and digital signals. Some of them are specialized for connecting to digital TTL microcircuits (597CA2), some - to digital ESL microcircuits (597CA1), but most are so-called. "comparators for general use" (LM393/LM339/K554CA3/K597CA3). Their main difference from the op amps lies in the special device of the output stage, which is made on an open collector transistor (Fig. 16).


Rice. 16 Comparator output stage for general applications
and its connection to the load resistor

This requires the mandatory use of an external load resistor(R1), without which the output signal is simply physically unable to form a high (positive) output level. The voltage +U2 to which the load resistor is connected may be different from the supply voltage +U1 of the comparator chip itself. This allows simple means to provide the desired output level - be it TTL or CMOS.

Note

In most comparators, an example of which can be dual LM393 (LM193 / LM293) or exactly the same in circuitry, but quad LM339 (LM139 / LM239), the emitter of the output stage transistor is connected to the negative power terminal, which somewhat limits their scope. In this regard, I would like to draw attention to the comparator LM31 (LM111 / LM211), the analogue of which is the domestic 521 / 554CA3, in which both the collector and the emitter of the output transistor are separately output, which can be connected to other voltages than the supply voltage of the comparator itself. Its only and relative disadvantage is that it is only one in an 8-pin (sometimes 14-pin) package.

So far, we have considered circuits in which the input signal was fed to the input(s) through Rin, i.e. they were all converters input voltage in day off voltage same. In this case, the input current flowed through Rin. What happens if its resistance is taken equal to zero? The circuit will work in exactly the same way as the inverting amplifier discussed above, only the output impedance of the signal source (Rout) will serve as Rin, and we get converter input current in day off voltage(Fig. 17).


Rice. 17 Scheme of the current-to-voltage converter at the op-amp

Since the potential at the inverting input is the same as at the non-inverting one (in this case it is "virtual zero"), the entire input current ( Iin) will flow through Rooc between the output of the signal source (G) and the output of the op-amp. The input resistance of such a circuit is close to zero, which makes it possible to build micro/milliammeters on its basis, which practically do not affect the current flowing through the measured circuit. Perhaps the only limitation is the permissible input voltage range of the op-amp, which should not be exceeded. It can also be used to build, for example, a linear photodiode current-to-voltage converter and many other circuits.

We have considered the basic principles of operation of the OS in various schemes for its inclusion. One important question remains: nutrition.

As mentioned above, an op amp typically has only 5 pins: two inputs, an output, and two power pins, positive and negative. In the general case, bipolar power is used, that is, the power supply has three outputs with potentials: + U; 0; -U.

Once again, carefully consider all the above figures and see that a separate output of the midpoint in the op-amp NO ! It is simply not needed for their internal circuitry to work. In some circuits, a non-inverting input was connected to the midpoint, however, this is not the rule.

Consequently, overwhelming majority modern op amps are designed to power UNIPOLAR voltage! A logical question arises: “Why then do we need bipolar power,” if we depicted it so stubbornly and with enviable constancy in the drawings?

It turns out it's just very comfortably for practical purposes for the following reasons:

A) To ensure sufficient current and output voltage swing through the load (Fig. 18).


Rice. 18 The flow of output current through the load with various options for supplying the op-amp

For now, we will not consider the input (and OOS) circuits of the circuits shown in the figure (“black box”). Let's take it for granted that some input sinusoidal signal is applied to the input (black sinusoid on the graphs) and the output is the same sinusoidal signal, amplified with respect to the input colored sinusoid on the graphs).

When connecting the load Rload. between the output of the op-amp and the midpoint of the connection of the power supplies (GB1 and GB2) - Fig. 18, A, the current flows through the load symmetrically about the midpoint (respectively, the red and blue half-waves), and its amplitude is maximum and the voltage amplitude at Rload. also the maximum possible - it can reach almost supply voltages. The current from the power source of the corresponding polarity is closed through the OS, Rload. and a power source (red and blue lines showing current flow in the corresponding direction).

Since the internal resistance of the op-amp power supplies is very low, the current through the load is only limited by its resistance and the op-amp's maximum output current, which is typically 25 mA.

When the op-amp is powered by a unipolar voltage as common bus the negative (negative) pole of the power source is usually selected, to which the second output of the load is connected (Fig. 18, B). Now the current through the load can only flow in one direction (shown by the red line), the second direction simply has nowhere to come from. In other words, the current through the load becomes asymmetrical (pulsating).

It is impossible to say unequivocally that this option is bad. If the load is, say, a dynamic head, then for it it is bad unambiguously. However, there are many applications where connecting a load between the output of the op-amp and one of the power rails (usually negative polarity) is not only acceptable, but also the only possible one.

If, nevertheless, it is necessary to ensure the symmetry of the current flow through the load with a unipolar supply, then it is necessary to galvanically decouple it from the output of the op-amp with a galvanic capacitor C1 (Fig. 18, B).

B) To ensure the required current of the inverting input, as well as bindings input signals to some arbitrarily selected level accepted for the reference (zero) - setting the operation mode of the OS for direct current (Fig. 19).


Rice. 19 Connecting the input signal source with various options for supplying the op-amp

Now consider the options for connecting input signal sources, excluding from consideration the connection of the load.

Connecting the inverting and non-inverting inputs to the midpoint of the power supply connection (Fig. 19, A) was considered when analyzing the previously given diagrams. If the non-inverting input draws no current and simply accepts the mid-point potential, then through the signal source (G) and Rin connected in series, the current flows, closing through the corresponding power source! And since their internal resistances are negligible compared to the input current (many orders of magnitude less than Rin), it practically does not affect the supply voltage.

Thus, with a unipolar supply of the op-amp, you can quite easily form the potential supplied to its non-inverting input using the divider R1R2 (Fig. 19, B, C). Typical resistor values ​​of this divider are 10 ... 100 kOhm, and it is highly desirable to shunt the lower one (connected to a common negative bus) with a 10 ... 22 microfarad capacitor in order to significantly reduce the effect of supply voltage ripples on the potential of such artificial middle point.

But it is extremely undesirable to connect the signal source (G) to this artificial midpoint because of the same input current. Let's guess. Even with the ratings of the divider R1R2 = 10 kOhm and Rin = 10…100 kOhm, the input current Iin will be at best 1/10, and at worst - up to 100% of the current passing through the divider. Consequently, the potential at the non-inverting input will “float” by the same amount in combination (in phase) with the input signal.

To eliminate the mutual influence of the inputs on each other when amplifying DC signals with such a connection, for the signal source it is necessary to organize a separate potential of the artificial midpoint, formed by resistors R3R4 (Fig. 19, B), or, if the AC signal is amplified, galvanically isolate the signal source from the inverting input by capacitor C2 (Fig. 19, B).

It should be noted that in the above diagrams (Fig. 18, 19) we assumed by default that the output signal should be symmetrical about either the midpoint of the power supplies or the artificial midpoint. In reality, this is not always necessary. Quite often, you want the output signal to have predominantly either positive or negative polarity. Therefore, it is not at all necessary that the positive and negative polarities of the power supply be equal in absolute value. One of them can be much smaller in absolute value than the other - only in such a way as to ensure the normal functioning of the OS.

A logical question arises: “Which one exactly?” To answer it, let's briefly consider the allowable voltage ranges of the input and output signals of the op-amp.

For any op amp, the output potential cannot be higher than the potential of the positive power rail and lower than the potential of the negative power rail. In other words, the output voltage cannot go beyond the limits of the supply voltages. For example, for an OPA277 op amp, the output voltage at a load resistance of 10 kΩ is 2 V less than the positive power rail and 0.5 V less than the negative power rail. The width of these "dead zones" of the output voltage, which the op amp output cannot reach, depends on the series factors such as output stage circuitry, load resistance, etc.). There are op amps that have minimal dead zones, for example, 50 mV to the supply rail voltage at a load of 10 kΩ (for OPA340), this feature of the op amp is called "rail-to-rail" (R2R).

On the other hand, for general-purpose op-amps, the input signals should also not exceed the supply voltage, and for some, be less than 1.5 ... 2 V. However, there are op-amps with specific input stage circuitry (for example, the same LM358 / LM324) , which can work not only from the negative power level, but even “negative” by 0.3 V, which greatly facilitates their use with unipolar power supply with a common negative bus.

Let's finally look at and feel these "spider bugs". You can even sniff and lick. I allow. Consider their most common options available to novice radio amateurs. Especially if you have to solder the op amp from the old equipment.

For op-amps of old designs, which necessarily require external circuits for frequency correction, in order to prevent self-excitation, it was typical to have additional conclusions. Because of this, some op amps did not even “fit” into an 8-pin package (Fig. 20, A) and were made in 12-pin round metal-glass, for example, K140UD1, K140UD2, K140UD5 (Fig. 20, B) or in 14-pin DIP packages, for example, K140UD20, K157UD2 (Fig. 20, B). The abbreviation DIP is an abbreviation of the English expression "Dual In line Package" and translates as "double-sided package".

The round metal-glass case (Fig. 20, A, B) was used as the main one for imported op-amps until about the mid-70s, and for domestic op-amps - until the mid-80s and is now used for the so-called. "military" applications ("5th acceptance").

Sometimes domestic op-amps were placed in currently rather “exotic” cases: a 15-pin rectangular metal-glass for the hybrid K284UD1 (Fig. 20, D), in which the key is an additional 15th pin from the case, and others. True, I personally have not met planar 14-pin packages (Fig. 20, E) for placing an op-amp in them. They were used for digital circuits.


Rice. 20 Cases of domestic operational amplifiers

Modern op amps, for the most part, contain correction circuits right on the chip, which made it possible to get by with a minimum number of pins (as an example, a 5-pin SOT23-5 for a single op amp - Fig. 23). This made it possible to place two to four completely independent (except for common power outputs) op-amps made on a single chip in one case.


Rice. 21 Two-row plastic cases of modern op amps for output mounting (DIP)

Sometimes you can find op-amps placed in single-row 8-pin (Fig. 22) or 9-pin packages (SIP) - K1005UD1. The abbreviation SIP is an abbreviation of the English expression "Single In line Package" and translates as "housing with one-way pinout."


Rice. 22 Single-row plastic case of double op-amps for through-hole mounting (SIP-8)

They were designed to minimize the space occupied on the board, but, unfortunately, they were "late": by this time, surface mount packages (SMD - Surface Mounting Device) by soldering directly to the board tracks (Fig. 23) had become widespread. However, for beginners, their use presents significant difficulties.


Rice. 23 Cases of modern imported op amps for surface mounting (SMD)

Very often, the same microcircuit can be "packed" by the manufacturer in different packages (Fig. 24).


Rice. 24 Placement options for the same chip in different packages

The conclusions of all microcircuits have a sequential numbering, counted from the so-called. "key", indicating the location of the output at number 1. (Fig. 25). IN any if the body is positioned with terminals Push, their numbering goes in ascending order against clockwise!


Rice. 25 Pin assignment of operational amplifiers
in various cases (pinout), top view;
numbering direction shown by arrows

In round metal-glass cases, the key has the form of a side protrusion (Fig. 25, A, B). Here, from the location of this key, huge “rakes” are possible! In domestic 8-pin cases (302.8), the key is located opposite the first pin (Fig. 25, A), and in imported TO-5 - opposite the eighth pin (Fig. 25, B). In 12-pin cases, both domestic (302.12) and imported, the key is located between the first and 12th conclusions.

Typically, the inverting input, both in round glass-metal and DIP packages, is connected to the 2nd pin, the non-inverting input to the 3rd pin, the output to the 6th pin, the power minus to the 4th pin, and the power plus to the pin 4. 7th. However, there are exceptions (another possible "rake"!) In the pinout of the OU K140UD8, K574UD1. In them, the numbering of the conclusions is shifted by one counterclockwise compared to the generally accepted for most other types, i.e. they are connected to the terminals, as in imported cases (Fig. 25, B), and the numbering corresponds to domestic ones (Fig. 25, A).

In recent years, most of the OS "domestic purposes" began to be placed in plastic cases (Fig. 21, 25, C-D). In these cases, the key is either a recess (dot) opposite the first pin, or a cutout in the end of the case between the first and 8th (DIP-8) or 14th (DIP-14) pins, or a chamfer along the first half of the pins (Fig. 21, middle). The pin numbering in these cases also goes against clockwise when viewed from above (with conclusions away from you).

As mentioned above, internally corrected op amps have a total of five outputs, of which only three (two inputs and an output) belong to each individual op amp. This made it possible to place two completely independent (with the exception of plus and minus power, which require two more pins) op amps on one chip in one 8-pin package (Fig. 25, D), and even four in a 14-pin package (Fig. 25, D). As a result, at present, most op-amps are produced at least dual, for example, TL062, TL072, TL082, cheap and simple LM358, etc. Exactly the same in internal structure, but quad - respectively, TL064, TL074, TL084 and LM324.

With regard to the domestic analogue of the LM324 (K1401UD2), there is one more “rake”: if in the LM324 the plus of the power supply is connected to the 4th pin, and the minus to the 11th, then in K1401UD2 it is the other way around: the plus of the power is brought to the 11th pin, and minus - on the 4th. However, this difference does not cause any difficulties with wiring. Since the pinout of the op-amp pins is completely symmetrical (Fig. 25, E), you just need to turn the case 180 degrees so that the 1st pin takes the place of the 8th. Yes, that's all.

A few words about the labeling of imported OUs (and not only OUs). For a number of developments of the first 300 digital designations, it was customary to designate the quality group with the first digit of the digital code. For example, LM158/LM258/LM358 op amps, LM193/LM293/LM393 comparators, TL117/TL217/TL317 adjustable three-pin stabilizers, etc. are completely identical in internal structure, but differ in temperature operating range. For LM158 (TL117) the operating temperature range is from minus 55 to +125 ... 150 degrees Celsius (the so-called "combat" or military range), for LM258 (TL217) - from minus 40 to +85 degrees ("industrial" range) and for LM358 (TL317) - from 0 to +70 degrees ("household" range). At the same time, the price for them may be completely inappropriate for such a gradation, or differ very slightly ( inscrutable ways of pricing!). So you can buy them with any marking available “for the pocket” of a beginner, without particularly chasing the first “troika”.

After the first three hundred digital markings were exhausted, reliability groups began to be marked with letters, the meaning of which is deciphered in datasheets (Datasheet literally translates as “data table”) for these components.

Conclusion

So we studied the "alphabet" of the operation of the op-amp, capturing a little and comparators. Next, you need to learn how to add words, sentences and whole meaningful “compositions” (workable schemes) from these “letters”.

Unfortunately, "It is impossible to grasp the immensity." If the material presented in this article helped to understand how these "black boxes" work, then further deepening into the analysis of their "stuffing", the influence of input, output and transient characteristics, is the task of a more advanced study. Information about this is described in detail and thoroughly in a variety of existing literature. As grandfather William of Ockham used to say: "Entities should not be multiplied beyond what is necessary." There is no need to repeat what has already been well described. All you need to do is not be lazy and read it.


11. http://www.texnic.ru/tools/lekcii/electronika/l6/lek_6.html

Therefore, let me take my leave, with respect, etc., the author Alexey Sokolyuk ()