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RF amplifier circuit. Amplifiers of radio frequency and intermediate frequency of the radio receiver

RADIO FREQUENCY AND INTERMEDIATE FREQUENCY AMPLIFIERS

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Article subject:	RADIO FREQUENCY AND INTERMEDIATE FREQUENCY AMPLIFIERS
Rubric (thematic category)	Connection

The amplification of the received radio signals in the receiving device is carried out in its preselector, ᴛ.ᴇ. at radio frequency, and after the frequency converter - at an intermediate frequency. Accordingly, radio frequency amplifiers (URCH) and intermediate frequency amplifiers (IFA) are distinguished. In these amplifiers, together with the gain, the frequency selectivity of the receiver must be ensured. To do this, the amplifiers contain resonant circuits: single oscillatory circuits, filters on coupled circuits, various types of lumped selectivity filters. RF amplifiers with variable tuning are usually made with a selective system similar to that used in the input circuit of the receiver, most often these are single-loop selective circuits.

In intermediate frequency amplifiers, complex types of selective systems with a frequency response close to rectangular, such as electromechanical filters, are used. ( EMF ), quartz filters (CF), filters on surface (bulk) acoustic waves (SAW, SOW), etc.

Most modern receivers use single-stage RF. Less commonly, with high requirements for selectivity and noise figure, the RF amplifier may contain up to three stages.

The basic electrical characteristics of amplifiers include:

1.Resonant voltage gain .

At ultrahigh frequencies (SHF), the concept of power gain is more often used, where - the active component of the input conductance of the amplifier; - active component of load conductivity.

2. Frequency selectivity of the amplifier shows the relative reduction in gain for a given detuning.

Sometimes selectivity is characterized by a coefficient of squareness, for example, .

3. Noise factor determines the noise properties of the amplifier.

4. Signal distortion in the amplifier: amplitude-frequency, phase, non-linear.

5. The stability of the amplifier is determined by its ability to maintain the main characteristics during operation (usually K o and frequency response), as well as the absence of a tendency to self-excitation.

Figures 1-3 show the main schemes of the UFC, and in Figure 4 the scheme of the IF with a selectivity concentration filter (FSI) in the form of an electromechanical filter.

Fig.1. URC on a field-effect transistor

Fig.2. URC on a bipolar transistor

Fig.3. URCH with inductive coupling with the electoral system

Fig.4. IF with a lumped selectivity filter

In radio frequency and intermediate frequency amplifiers, two options for switching on an amplifying device are mainly used: with a common emitter (common source) and a cascode transistor switching circuit.

Figure 1 shows a common-source FET amplifier circuit. An oscillatory circuit is included in the drain circuit L K S K. The circuit is tuned by capacitor C To(can be used to adjust the contour of the varicap or varicap matrix).

The amplifier uses a series drain power supply through a filter R3C3. Gate bias voltage VT1 determined by the voltage drop from the source current across the resistor R2. Resistor R1 is the leakage resistance of the transistor VT1 and serves to transfer the bias voltage to the gate of the transistor.

On fig. 2 shows a similar diagram of the URF on a bipolar transistor. Here, a double incomplete switching on of the circuit with transistors VT1, VT2 is used, which makes it possible to ensure the extremely important shunting of the circuit from the output side of the transistor VT1 and from the input side of the transistor VT2 . The supply voltage to the collector of the transistor is applied through the filter R4C4 and part of the turns of the circuit coil L K. DC mode and temperature stabilization is provided by resistors R1, R2 and R3. Capacity C2 eliminates negative AC feedback.

On fig. 3 shows a circuit with a transformer connection of the circuit with the collector of the transistor and an autotransformer connection with the input of the next stage. Usually, in this case, the "extended" contour setting is used (see lab work No. 1).

On fig. 4 shows a diagram of a cascade of an IF with FSI, made on a 265 UVZ microcircuit . The microcircuit is a cascode amplifier OE - OB.

IF amplifiers provide the main gain and selectivity of the receiver in the adjacent channel. Them important feature is that they operate at a fixed intermediate frequency and have a large gain, of the order of magnitude.

When using various types of FSI, the required amplification of the IF is achieved by using broadband cascades.

Common to all schemes is the double incomplete inclusion of the electoral system. (Full inclusion can be considered as a special case when the transformation coefficients m and n are equal to one). For this reason, one generalized equivalent equivalent circuit of the amplifier can be used for analysis (see Fig. 5).

Fig.5. Generalized equivalent circuit of a resonant amplifier

In the diagram, the transistor on the output side is replaced by an equivalent current generator with parameters, and current, and on the input side of the next stage with conductivity, . Leakage resistor R4 (Fig. 1) or divider (Fig. 2) are replaced by conduction (or).

Usually, the sum of the conductivities is considered to be the conductance of the load GH, ᴛ.ᴇ.

An analysis of the equivalent circuit makes it possible to obtain all the calculated ratios for determining the characteristics of the cascade.

So, the complex gain of the cascade is determined by the expression

equivalent resonant conductivity of the circuit;

Generalized detuning of the contour.

From this relation it is easy to determine the modulus of the coefficient

amplification

and resonant gain of the cascade URF

The resonant gain reaches its maximum value with the same circuit shunting on the output side of the active device and on the load side (input of the next stage), ᴛ.ᴇ. when

The above relations allow us to obtain the equation of the resonant curve of the amplifier. So, for small detunings, . From where, the bandwidth of the URF at a level of 0.707 (- 3dB) is equal to

The resonant gain of the single-circuit cascade of the IF is the same as that of the single-circuit UFC

For an IF with a two-loop bandpass filter, the resonant gain of the stage is given by

where - the factor of connection between contours, and - coefficient of connection between contours.

The gain (in terms of voltage) of the IF with any FSI when matching the filter at the input and output must be calculated by the formula

Here, are the characteristic (wave) impedances of the FSI at the input and output, respectively;

Filter gain in the transparency (transmission) band.

In the event that the filter attenuation in the transparency band is known in decibels, then

Inclusion factors m and n calculated from the filter matching condition at the input and output

The resonant characteristic of the IF cascade with FSI is completely determined by the curve of the change in the transfer coefficient FSI from frequency. Individual points of the resonance curve FSI specified in reference books.

The gain of the selective amplifier should not exceed the value of the stable gain. In general, it can be estimated from the expression

If a cascode circuit is used as an amplifying element, then it is extremely important to substitute the corresponding conductivities for the cascode circuit, for example, for the OE circuit - OB

In the case of using field-effect transistors, the active component of conductivity can be neglected and

AMPLIFIERS OF RADIO FREQUENCY AND INTERMEDIATE FREQUENCY OF RADIO RECEPTION DEVICE - concept and types. Classification and features of the category "RADIO FREQUENCY AND INTERMEDIATE FREQUENCY AMPLIFIERS OF RADIO RECEPTION DEVICE" 2017, 2018.

High frequency amplifiers (UHF) are used to increase the sensitivity of radio receivers - radios, televisions, radio transmitters. Placed between the receiving antenna and the input of a radio or television receiver, these UHF circuits amplify the signal coming from the antenna (antenna amplifiers).

The use of such amplifiers allows you to increase the radius of reliable radio reception, in the case of radio stations (transceivers - transceivers) either increase the operating range, or, while maintaining the same range, reduce the radiation power of the radio transmitter.

Figure 1 shows examples of UHF schemes often used to increase the sensitivity of radio equipment. The values of the elements used depend on the specific conditions: on the frequencies (lower and upper) of the radio band, on the antenna, on the parameters of the subsequent cascade, on the supply voltage, etc.

Figure 1 (a) shows broadband UHF circuit according to the scheme with a common emitter(OE). Depending on the transistor used, this circuit can be successfully applied up to frequencies of hundreds of megahertz.

It should be recalled that in the reference data for transistors, limiting frequency parameters are given. It is known that when assessing the frequency capabilities of a transistor for a generator, it is enough to focus on the limiting value of the operating frequency, which should be at least two to three times lower than the limiting frequency indicated in the passport. However, for an RF amplifier connected according to the OE scheme, the limiting passport frequency already needs to be reduced by at least an order of magnitude or more.

Fig.1. Examples of circuits of simple high-frequency amplifiers (UHF) on transistors.

Radio elements for the circuit in Fig. 1 (a):

R1=51k(for silicon transistors), R2=470, R3=100, R4=30-100;
C1=10-20, C2=10-50, C3=10-20, C4=500-Zn;

Capacitor values are given for VHF frequencies. Capacitors such as KLS, KM, KD, etc.

Transistor stages, as is known, connected according to the common emitter (CE) circuit, provide a relatively high gain, but their frequency properties are relatively low.

Transistor stages connected in a common base (CB) circuit have less gain than OE transistor circuits, but their frequency properties are better. This allows you to use the same transistors as in OE circuits, but at higher frequencies.

Figure 1 (b) shows broadband high frequency (UHF) amplifier circuit on a single transistor according to the scheme with a common base. In the collector circuit (load), the LC circuit is turned on. Depending on the transistor used, this circuit can be successfully applied up to frequencies of hundreds of megahertz.

Radio elements for the circuit in Fig. 1 (b):

R1=1k, R2=10k. R3=15k, R4=51 (for supply voltage ZV-5V). R4=500-3 k (for supply voltage 6V-15V);
C1=10-20, C2=10-20, C3=1n, C4=1n-3n;
T1 - silicon or germanium RF transistors, for example. KT315. KT3102, KT368, KT325, GT311, etc.

Capacitor and circuit values are given for VHF frequencies. Capacitors such as KLS, KM, KD, etc.

Coil L1 contains 6-8 turns of PEV 0.51 wire, brass cores 8 mm long with M3 thread, tap from 1/3 of the turns.

Figure 1 (c) shows another broadband scheme UHF on one transistor, included according to the scheme with a common base. An RF inductor is included in the collector circuit. Depending on the transistor used, this circuit can be successfully applied up to frequencies of hundreds of megahertz.

Radioelements:

R1=1k, R2=33k, R3=20k, R4=2k (for supply voltage 6V);
C1=1n, C2=1n, C3=10n, C4=10n-33n;
T1 - silicon or germanium RF transistors, for example, KT315, KT3102, KT368, KT325, GT311, etc.

Capacitor and circuit values are given for MW, HF frequencies. For higher frequencies, such as the VHF band, the capacitance values must be reduced. In this case, chokes D01 can be used.

Capacitors such as KLS, KM, KD, etc.

Coils L1 - chokes, for the SV range it can be coils on rings 600NN-8-K7x4x2, 300 turns of PEL 0.1 wire.

Larger gain value can be obtained through the use multi-transistor circuits. These can be various circuits, for example, based on the OK-OB cascode amplifier based on transistors of different structures with series supply. One of the options for such a UHF scheme is shown in Fig. 1 (d).

This UHF scheme has significant amplification (tens and even hundreds of times), but cascode amplifiers cannot provide significant amplification at high frequencies. Such schemes, as a rule, are used at the frequencies of the LW and MW bands. However, with the use of microwave transistors and careful design, such circuits can be successfully used up to frequencies of tens of megahertz.

Radioelements:

R1=33k, R2=33k, R3=39k, R4=1k, R5=91, R6=2.2k;
C1=10n, C2=100, C3=10n, C4=10n-33n. C5=10n;
T1 - GT311, KT315, KT3102, KT368, KT325, etc.
T2 - GT313, KT361, KT3107, etc.

Capacitor and circuit values are for MW frequencies. For higher frequencies, such as the HF band, the capacitance values and the loop inductance (number of turns) must be reduced accordingly.

Capacitors such as KLS, KM, KD, etc. Coil L1 - for the MW range contains 150 turns of PELSHO 0.1 wire on 7 mm frames, trimmers M600NN-3-SS2.8x12.

When setting up the circuit in Fig. 1 (d), it is necessary to select resistors R1, R3 so that the voltages between the emitters and collectors of the transistors become the same and amount to 3V at a circuit supply voltage of 9 V.

The use of transistorized UHF makes it possible to amplify radio signals. coming from antennas, in television ranges - meter and decimeter waves. In this case, antenna amplifier circuits built on the basis of circuit 1(a) are most often used.

Antenna amplifier circuit example for frequency range 150-210 MHz shown in Fig. 2 (a).

Fig.2.2. Scheme of the antenna amplifier of the MV range.

Radioelements:

R1=47k, R2=470, R3=110, R4=47k, R5=470, R6=110. R7=47k, R8=470, R9=110, R10=75;
C1=15, C2=1n, C3=15, C4=22, C5=15, C6=22, C7=15, C8=22;
T1, T2, TZ - 1T311(D, L), GT311D, GT341 or similar.

Capacitors such as KM, KD, etc. The frequency band of this antenna amplifier can be expanded in the low-frequency region by a corresponding increase in the capacitances that make up the circuit.

Radio elements for the antenna amplifier option for the range 50-210 MHz:

R1=47k, R2=470, R3=110, R4=47k, R5=470, R6=110. R7=47k, R8=470. R9=110, R10=75;
C1=47, C2=1n, C3=47, C4=68, C5=47, C6=68, C7=47, C8=68;
T1, T2, TZ - GT311A, GT341 or similar.

Capacitors such as KM, KD, etc. When repeating this device, all requirements must be met. required for installation of high-frequency structures: minimum lengths of connecting conductors, shielding, etc.

An antenna amplifier designed for use in the ranges of television signals (and higher frequencies) can be overloaded with signals from powerful MW, HF, VHF radio stations. Therefore, a wide bandwidth may not be optimal, since this may interfere with the normal operation of the amplifier. This is especially true in the lower region of the operating range of the amplifier.

For the circuit of the reduced antenna amplifier, this can be significant, because the slope of the gain decay in the lower part of the range is relatively low.

You can increase the steepness of the amplitude-frequency characteristic (AFC) of this antenna amplifier by using 3rd order high pass filter. To do this, an additional LC circuit can be used at the input of this amplifier.

The diagram for connecting an additional LC high-pass filter to the antenna amplifier is shown in fig. 2(b).

Additional filter parameters (indicative):

C=5-10;
L - 3-5 turns of PEV-2 0.6. winding diameter 4 mm.

It is advisable to adjust the frequency band and shape of the frequency response using appropriate measuring instruments (sweep frequency generator, etc.). The shape of the frequency response can be adjusted by changing the values of capacitances C, C1, changing the pitch between turns L1 and the number of turns.

Using the described circuit solutions and modern high-frequency transistors (microwave transistors - microwave transistors), you can build an antenna amplifier for the UHF range. This amplifier can be used both with a UHF radio receiver, for example, part of a VHF radio station, or in conjunction with a TV.

Figure 3 shows UHF antenna amplifier circuit.

Fig.3. UHF antenna amplifier circuit and connection diagram.

The main parameters of the UHF range amplifier:

Frequency band 470-790 MHz,
Gain - 30 dB,
Noise figure -3 dB,
Input and output resistance - 75 Ohm,
Consumption current - 12 mA.

One of the features of this circuit is the supply voltage to the antenna amplifier circuit through the output cable, through which the output signal is supplied from the antenna amplifier to the radio signal receiver - a VHF radio receiver, for example, a VHF radio receiver or TV.

The antenna amplifier consists of two transistor stages connected according to a common emitter circuit. At the input of the antenna amplifier, a 3rd order high-pass filter is provided, which limits the operating frequency range from below. This increases the noise immunity of the antenna amplifier.

Radioelements:

R1=150k, R2=1k, R3=75k, R4=680;
C1=3.3, C10=10, C3=100, C4=6800, C5=100;
T1, T2 - KT3101A-2, KT3115A-2, KT3132A-2.
Capacitors C1, C2 type KD-1, the rest - KM-5 or K10-17v.
L1 - PEV-2 0.8 mm, 2.5 turns, winding diameter 4 mm.
L2 - RF choke, 25 µH.

Figure 3 (b) shows the connection diagram of the antenna amplifier to the antenna jack of the TV receiver (to the UHF band selector) and to the remote 12 V power supply. In this case, as can be seen from the diagram, power is supplied to the circuit through the coaxial cable used and for transmitting an amplified UHF radio signal from an antenna amplifier to a receiver - a VHF radio or a TV.

Connection radio elements, Fig. 3 (b):

C5=100;
L3 - RF choke, 100 uH.

The installation is carried out on a double-sided fiberglass SF-2 by a hinged method, the length of the conductors and the area of the contact pads are minimal, it is necessary to provide for a thorough shielding of the device.

Establishing an amplifier is reduced to setting the collector currents of transistors and are regulated using R1 and R3, T1 - 3.5 mA, T2 - 8 mA; the shape of the frequency response can be adjusted by selecting C2 within 3-10 pF and changing the pitch between the turns of L1.

Literature: Rudomedov E.A., Rudometov V.E. - Electronics and espionage passions-3.

Since the radio frequency amplifier is located at the input of the radio receiver, its noise characteristics mainly determine the characteristics of the entire device as a whole. It is the noise figure of the RF amplifier that determines . The non-linear properties of the amplifier are evaluated by the characteristics IP2 and IP3. To ensure high linearity in all stages of the receiver are used. The dot is a very important parameter.

In connection with the microminiaturization of the modern element base and the associated miniaturization of the nodes of the radio receiver, it is now possible to use circuit solutions on the microwave that were previously used at much lower frequencies. This is due to the fact that the dimensions of the block relative to the wavelength of the working oscillation become less than one tenth of the wavelength, and as a result, when developing this block, wave effects during the propagation of oscillations can be neglected.

An additional increase in the stability of the circuit is achieved by including low-pass filters at the input and output of the transistor stage. These filters are calculated for the entire frequency band in which the transistor retains amplifying properties. As a result, phase balance is not maintained over the entire frequency range and self-excitation becomes impossible. The same filter converts the input and output resistance of the transistor to a standard resistance of 50 ohms. The input and output capacitance is included in the filter. RF amplifier with matching circuits at the input and output is shown in Figure 1.

Figure 1. Schematic diagram of an RF amplifier with an input and output impedance of 50 ohms on a common base transistor

In this circuit, R1 ... R3 is implemented in direct current. Capacitor C2 provides grounding of the base of the transistor at high frequency, and capacitor C3 filters the power circuits from interference. Inductor L2 is the collector load of transistor VT1. It passes the supply current into the collector circuit VT1, but at the same time decouples the power supply for alternating current of the radio frequency. Low-frequency filters L1, C1 and C4, L3 provide a transformation of the input and output resistance of the transistor to 50 ohms. The applied low-frequency filter circuit allows you to include in its composition the input or output capacitance of the transistor. The input capacitance of the transistor VT1, together with the capacitance C1, forms the input filter of the amplifier, and the output capacitance of the same transistor, together with the capacitance C4, forms the output low-frequency filter.

Another common RF amplifier circuit is the cascode amplifier circuit. In this scheme, two are connected in series - and with a common base. Such a solution makes it possible to further reduce the value of the pass capacitance of the amplifier. The most common cascode amplifier circuit is a circuit with galvanic coupling between transistor stages. An example of a cascode RF amplifier circuit assembled on bipolar transistors is shown in Figure 2.

Figure 2. Schematic diagram of a cascode RF amplifier

In this circuit, just like in the circuit shown in Figure 1, an emitter stabilization circuit for the operating point of the transistor VT2 is used. Capacitor C6 provides for the elimination of negative feedback at the frequency of the received signal. In some cases, this capacitor is not installed to increase the linearity of the amplifier and in order to reduce the gain of the radio frequency amplifier.

Capacitor C2 provides grounding of the base of the transistor VT1 for alternating current. Capacitor C4 filters the AC power supply. Resistors R1, R2, R3 determine the operating points of transistors VT1 and VT2. Capacitor C3 decouples the base circuit of the transistor VT2 for direct current from the previous stage (input bandpass filter). AC collector circuit load is inductor L2. As in the RF amplifier circuit with a common base, low-pass filters are applied at the input and output of the cascode amplifier. Their main purpose is to ensure the transformation of the input and output resistance to a value of 50 ohms.

Please note that three outputs of the circuit are enough to supply the input voltage and supply voltage, as well as remove the output amplified voltage. This allows you to make an amplifier in the form of a microcircuit with literally three leads. Such cases have minimal dimensions, and this makes it possible to avoid wave effects even at sufficiently high frequencies of the working signal.

Currently, radio frequency amplifier circuits are produced by a number of companies in the form of ready-made microcircuits. For example, we can name such microcircuits as RF3827, RF2360 from RFMD, ADL5521 from Analog Devises, MAALSS0038, AM50-0015 from M / A-COM. These microcircuits use gallium arsenide field-effect transistors. The upper amplified frequency can reach 3GHz. In this case, the noise figure ranges from 1.2 to 1.5 dB. An example of a circuit diagram of a radio frequency amplifier using an integrated circuit MAALSS0038 from M / A-COM is shown in Figure 3.

Figure 3. Schematic diagram of an RF amplifier using the integrated circuit MAALSS0038

RF signals in the range from hundreds of megahertz to units of gigahertz can only be amplified under the condition of very small dimensions of the microcircuits and careful study of the design of the printed circuit board. That is why all manufacturers of RF amplifiers give examples of printed circuit boards. An example of the design of a radio frequency amplifier printed circuit board assembled on a MAALSS0038 chip from M / A-COM is shown in Figure 4.

Figure 4. RF Amplifier PCB Design

It should be noted that often a filter similar to the input filter is often placed between the output of the RF amplifier and the input of the frequency converter, as shown in Figure 2. It allows you to increase the suppression of side channels generated in the frequency converter. Since the input impedance of the filter and the output impedance of the RF amplifier are 50 ohms, their coupling usually does not cause problems.

Literature:

Together with the article "Radio Frequency Amplifiers" they read:

With the simultaneous operation of the receiver and transmitter, questions arise about the electromagnetic compatibility of these nodes ...
http://website/WLL/Duplexer.php

When designing base station radio receivers, there is a requirement to distribute the signal energy from the antenna to the inputs of several radio receivers.
http://website/WLL/divider.php

The input filter is one of the most important components of the radio receiver...
The more complex the filter is applied as an input filter, the higher the quality of the radio can be obtained...
http://website/WLL/InFiltr/

The amplification of the received radio signals in the receiving device is carried out in its preselector, i.e. at radio frequency, and after the frequency converter - at an intermediate frequency. Accordingly, radio frequency amplifiers (URCH) and intermediate frequency amplifiers (IFA) are distinguished. In these amplifiers, together with the gain, the frequency selectivity of the receiver must be ensured. To do this, the amplifiers contain resonant circuits: single oscillatory circuits, filters on coupled circuits, various types of lumped selectivity filters. RF amplifiers with variable tuning are usually made with a selective system similar to that used in the input circuit of the receiver, most often these are single-loop selective circuits.

Most modern receivers use single-stage RF. Less commonly, with high requirements for selectivity and noise figure, the RF amplifier may contain up to three stages.

The main electrical characteristics of amplifiers include:

1.Resonant voltage gain .

At ultrahigh frequencies (SHF), the concept of power gain is more often used
, where
- the active component of the input conductance of the amplifier;
- active component of load conductivity.

2. Frequency selectivity of the amplifier shows the relative reduction in gain for a given detuning
.

Sometimes selectivity is characterized by a coefficient of rectangularity, for example,
.

3. Noise factor determines the noise properties of the amplifier.

4. Signal distortion in the amplifier: amplitude-frequency, phase, non-linear.

Figures 1-3 show the main schemes of the UFC, and in Figure 4 the scheme of the IF with a selectivity concentration filter (FSI) in the form of an electromechanical filter.

Fig.1. URC on a field-effect transistor

Fig.2. URC on a bipolar transistor

Fig.3. URCH with inductive coupling with the electoral system

Fig.4. IF with a lumped selectivity filter

Figure 1 shows a common-source FET amplifier circuit. An oscillatory circuit is included in the drain circuit L To FROM To . The circuit is tuned by capacitor C To(can be used to adjust the contour of the varicap or varicap matrix).

The amplifier uses a series drain power supply through a filter R3 C3 . Gate bias voltage VT1 determined by the voltage drop from the source current across the resistor R2 . Resistor R1 is the leakage resistance of the transistor VT1 and serves to transfer the bias voltage to the gate of the transistor.

On fig. 2 shows a similar diagram of the URF on a bipolar transistor. Here, a double incomplete inclusion of the circuit with transistors VT1, VT2, which allows you to provide the necessary shunting of the circuit from the output side of the transistor VT1 and from the input side of the transistor VT2 . The supply voltage to the collector of the transistor is applied through the filter R4C4 and part of the turns of the circuit coil L To . DC mode and temperature stabilization is provided by resistors R1, R2 and R3. Capacity C2 eliminates negative AC feedback.

On fig. 3 shows a circuit with a transformer connection of the circuit with the collector of the transistor and an autotransformer connection with the input of the next stage. Usually, in this case, an "extended" contour setting is used (see lab work No. 1).

On fig. 4 shows a diagram of a cascade of an IF with FSI, made on a 265 UVZ microcircuit . The microcircuit is a cascode amplifier OE - OB.

IF amplifiers provide the main amplification and selectivity of the receiver in the adjacent channel. Their important feature is that they operate at a fixed intermediate frequency and have a large gain, of the order
.

When using various types of FSI, the required amplification of the IF is achieved by using broadband cascades.

Common to all schemes is the double incomplete inclusion of the electoral system. (Full inclusion can be considered as a special case when the transformation coefficients m and n are equal to one). Therefore, one generalized equivalent equivalent circuit of the amplifier can be used for analysis (see Fig. 5).

Fig.5. Generalized equivalent circuit of a resonant amplifier

In the diagram, the transistor on the output side is replaced by an equivalent current generator with parameters
,
and current
, and from the input side of the next cascade by conductivity
,
. Leakage resistor R4 (fig. 1) or divider
(Fig. 2) are replaced by conductivity
(
or
).

Usually the sum of the conductivities
considered the conductivity of the load GH, i.e.

An analysis of the equivalent circuit makes it possible to obtain all the calculated ratios for determining the characteristics of the cascade.

So, the complex gain of the cascade is determined by the expression

, where -

equivalent resonant conductivity of the circuit;

Generalized detuning of the contour.

From this relation it is easy to determine the modulus of the coefficient

amplification

and resonant gain of the cascade URF

The resonant gain reaches its maximum value with the same shunting of the circuit from the output side of the active device and from the load side (input of the next stage), i.e. when

The above relations allow us to obtain the equation of the resonant curve of the amplifier. So, with small detunings,
. From where, URF bandwidth level 0.707 (- 3dB) is equal to

The resonant gain of the single-circuit cascade of the IF is the same as that of the single-circuit UFC

For an IF with a two-loop bandpass filter, the resonant gain of the stage is given by

where
- the coupling factor between the contours, and - coupling coefficient between circuits.

The gain (in terms of voltage) of the IF with any FSI when matching the filter at the input and output can be calculated by the formula

Here
,
- characteristic (wave) resistances of the FSI at the input and output, respectively;

- filter transfer coefficient in the transparency (transmission) band.

In the event that the attenuation of the filter in the transparency band is known in decibels, then

Inclusion factors m and n calculated from the filter matching condition at the input and output

,
.

The gain of the selective amplifier must not exceed the value of the stable gain
. In general,
can be estimated from the expression

If a cascode circuit is used as an amplifying element, then it is necessary to substitute the corresponding conductivities for the cascode circuit, for example, for the OE - OB circuit

In the case of using field-effect transistors, the active component of conductivity can be neglected and

Fig.1. Examples of circuits of simple high-frequency amplifiers (UHF) on transistors.

Radio elements for the circuit in Fig. 1 (a):

R1=51k(for silicon transistors), R2=470, R3=100, R4=30-100;
C1=10-20, C2=10-50, C3=10-20, C4=500-Zn;

Capacitor values are given for VHF frequencies. Capacitors such as KLS, KM, KD, etc.

Transistor stages, as is known, connected according to the common emitter (CE) circuit, provide a relatively high gain, but their frequency properties are relatively low.

Radio elements for the circuit in Fig. 1 (b):

R1=1k, R2=10k. R3=15k, R4=51 (for supply voltage ZV-5V). R4=500-3 k (for supply voltage 6V-15V);
C1=10-20, C2=10-20, C3=1n, C4=1n-3n;
T1 - silicon or germanium RF transistors, for example. KT315. KT3102, KT368, KT325, GT311, etc.

Capacitor and circuit values are given for VHF frequencies. Capacitors such as KLS, KM, KD, etc.

Coil L1 contains 6-8 turns of PEV 0.51 wire, brass cores 8 mm long with M3 thread, tap from 1/3 of the turns.

Radioelements:

R1=1k, R2=33k, R3=20k, R4=2k (for supply voltage 6V);
C1=1n, C2=1n, C3=10n, C4=10n-33n;
T1 - silicon or germanium RF transistors, for example, KT315, KT3102, KT368, KT325, GT311, etc.

Capacitor and circuit values are given for MW, HF frequencies. For higher frequencies, such as the VHF band, the capacitance values must be reduced. In this case, chokes D01 can be used.

Capacitors such as KLS, KM, KD, etc.

Coils L1 - chokes, for the SV range it can be coils on rings 600NN-8-K7x4x2, 300 turns of PEL 0.1 wire.

Radioelements:

R1=33k, R2=33k, R3=39k, R4=1k, R5=91, R6=2.2k;
C1=10n, C2=100, C3=10n, C4=10n-33n. C5=10n;
T1 - GT311, KT315, KT3102, KT368, KT325, etc.
T2 - GT313, KT361, KT3107, etc.

Capacitor and circuit values are for MW frequencies. For higher frequencies, such as the HF band, the capacitance values and the loop inductance (number of turns) must be reduced accordingly.

Capacitors such as KLS, KM, KD, etc. Coil L1 - for the MW range contains 150 turns of PELSHO 0.1 wire on 7 mm frames, trimmers M600NN-3-SS2.8x12.

Antenna amplifier circuit example for frequency range 150-210 MHz shown in Fig. 2 (a).

Fig.2.2. Scheme of the antenna amplifier of the MV range.

Radioelements:

R1=47k, R2=470, R3=110, R4=47k, R5=470, R6=110. R7=47k, R8=470, R9=110, R10=75;
C1=15, C2=1n, C3=15, C4=22, C5=15, C6=22, C7=15, C8=22;
T1, T2, TZ - 1T311(D, L), GT311D, GT341 or similar.

Capacitors such as KM, KD, etc. The frequency band of this antenna amplifier can be expanded in the low-frequency region by a corresponding increase in the capacitances that make up the circuit.

Radio elements for the antenna amplifier option for the range 50-210 MHz:

R1=47k, R2=470, R3=110, R4=47k, R5=470, R6=110. R7=47k, R8=470. R9=110, R10=75;
C1=47, C2=1n, C3=47, C4=68, C5=47, C6=68, C7=47, C8=68;
T1, T2, TZ - GT311A, GT341 or similar.

For the circuit of the reduced antenna amplifier, this can be significant, because the slope of the gain decay in the lower part of the range is relatively low.

The diagram for connecting an additional LC high-pass filter to the antenna amplifier is shown in fig. 2(b).

Additional filter parameters (indicative):

C=5-10;
L - 3-5 turns of PEV-2 0.6. winding diameter 4 mm.

Figure 3 shows UHF antenna amplifier circuit.

Fig.3. UHF antenna amplifier circuit and connection diagram.

The main parameters of the UHF range amplifier:

Frequency band 470-790 MHz,
Gain - 30 dB,
Noise figure -3 dB,
Input and output resistance - 75 Ohm,
Consumption current - 12 mA.

Radioelements:

R1=150k, R2=1k, R3=75k, R4=680;
C1=3.3, C10=10, C3=100, C4=6800, C5=100;
T1, T2 - KT3101A-2, KT3115A-2, KT3132A-2.
Capacitors C1, C2 type KD-1, the rest - KM-5 or K10-17v.
L1 - PEV-2 0.8 mm, 2.5 turns, winding diameter 4 mm.
L2 - RF choke, 25 µH.

Connection radio elements, Fig. 3 (b):

C5=100;
L3 - RF choke, 100 uH.

Literature: Rudomedov E.A., Rudometov V.E. - Electronics and espionage passions-3.

RF amplifier circuit. Amplifiers of radio frequency and intermediate frequency of the radio receiver

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