High frequency radio frequency amplifiers. RF and Intermediate Frequency Amplifiers RF Bandpass Amplifier with AGC

Since the radio frequency amplifier is located at the input of the radio receiver, its noise characteristics and dynamic range mainly determine the characteristics of the entire device. It is the noise figure of the radio frequency amplifier that determines the sensitivity of the radio receiver.

Signal amplification in the receiver can occur before the frequency converter, i.e. at the receiving frequency, and after the converter - at the intermediate frequency. Amplification at the frequency of the received signal is carried out using radio frequency amplifiers (RFA). In addition to amplification, frequency selectivity must also be ensured. Range amps must have variable tuning circuits. They are most often performed single-circuit. The active element of the amplifier is a field-effect or bipolar transistor in discrete or integrated design. In intermediate frequency amplifiers, preference is given to bipolar transistors due to their higher gain. RF amplifiers increase the selectivity of the mirror channel and the sensitivity of the receiver. According to the circuit structure, the RF frequency converters can be aperiodic or resonant.

Aperiodic RF frequencies They only increase the signal-to-noise ratio and the sensitivity of the receiver. They are most often used in direct amplification transistor receivers in the LW and SW ranges. A choke, resistor or transformer can serve as a load for aperiodic RF frequency converters. The resistor cascade of the URCH is easy to implement and configure. In transformer-based amplifiers, it is easier to match the output of one stage with the input of the next one. In addition, the transformer cascade of the RF amplifier can be easily converted into a reflex cascade.

Resonant amplifiers provide signal amplification and increase not only the real sensitivity, but also the selectivity of the mirror channel. Transistor resonant amplifiers in the DV, MV and HF ranges are assembled according to a circuit with OE, and in the VHF range - according to a circuit with OB.

AMP cascades may contain one or two resonant circuits. A single circuit RF amplifier provides less gain, but is easier to manufacture and configure. Circuits with inductive coupling of circuits allow you to change the connection and obtain the highest gain or better selectivity. By changing the connection over the range, you can somewhat compensate for the unevenness of the transmission coefficient of the input circuits.

VHF radio frequency amplifiers are made using cascade circuits. They have better characteristics than conventional URCHs. The first transistor is connected according to a circuit with an OE, due to which a low input conductivity of the amplifier is achieved, and the second V2 is connected according to a circuit with an OB, which provides a large stable gain. For direct current, the transistors are connected in series, which necessitates increasing the voltage of the power source.

In terms of gain, a cascode amplifier is equivalent to a single-stage amplifier with the forward conductivity of the first transistor and the load of the second. The cascode circuit is used in amplifiers of the meter wave range. It is advantageous to implement the first stage of the circuit on a field-effect transistor, which has a low noise level and low active input conductivity, while the selective system of the receiver, connected at the input of the cascode amplifier, will be less shunted. In the second stage, a drift transistor is preferred, connected according to the circuit with the OB and providing the highest stable gain. With this design of the amplifier's cascode circuit, its stable gain coefficient increases, the noise level is significantly reduced, and the selectivity of the receiver's radio signal path increases, which is their advantage.

Similar advantages are offered by cascade circuits (low noise level and high coefficient of stable gain) on electronic tubes, usually triodes, connected according to the common cathode - common grid circuit.

The amplification of received radio signals in the receiving device is carried out in its preselector, i.e. at radio frequency, and after the frequency converter - at intermediate frequency. Accordingly, a distinction is made between radio frequency amplifiers (RFA) and intermediate frequency amplifiers (IFA). In these amplifiers, along with the amplification, frequency selectivity of the receiver must be ensured. For this purpose, amplifiers contain resonant circuits: single oscillating circuits, filters on coupled circuits, various types of concentrated selectivity filters. Radio frequency amplifiers with variable tuning are usually made with a selective system similar to that used in input circuit

receiver, most often these are single-circuit selective circuits. ( Complex types of selective systems with frequency response close to rectangular, such as electromechanical filters, are used in intermediate frequency amplifiers. ), EMF

quartz filters (QF), filters based on surface (bulk) acoustic waves (SAW, SAW), etc.

Most modern receivers use single-stage amplifiers. Less commonly, with high requirements for selectivity and noise figure, AMPs can contain up to three stages. Among the main electrical characteristics

amplifiers include: .

1.Resonant voltage gain
, At ultrahigh frequencies (microwaves), the concept of power gain is more often used
- Where
active component of the input conductivity 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 squareness coefficient, for example, 3.Noise figure

determines the noise properties of the amplifier. 4. Signal distortion in the amplifier

: amplitude-frequency, phase, nonlinear. 5. Amplifier stability

is determined by its ability to maintain basic 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 circuits of the amplifier, and Figure 4 shows the circuit of the amplifier with a selectivity concentration filter (FSI) in the form of an electromechanical filter.

Fig.1. URCH on a field-effect transistor Fig.2. URCH on

bipolar transistor Fig.3. URCH with inductive coupling

with the electoral system

Fig.4.

Figure 1 shows a circuit of an amplifier based on a field-effect transistor with a common source. An oscillatory circuit is included in the drain circuit L TO L . WITH L The circuit is adjusted by capacitor C

(can be used to configure a varicap or varicap matrix circuit). The amplifier uses serial drain power through a filter3 R3 . C Gate Bias Voltage1 VT The amplifier uses serial drain power through a filter2 . determined by the voltage drop from the source current across the resistor The amplifier uses serial drain power through a filter1 Resistor Gate Bias Voltage1 is the leakage resistance of the transistor

and serves to transmit bias voltage to the gate of the transistor. In Fig. Figure 2 shows a similar circuit of the RF amplifier based on a bipolar transistor. Here, double incomplete inclusion of the circuit with transistors VT1, VT2, which makes it possible to provide the necessary bypassing of the circuit from the output side of transistor VT1 . and from the input side of transistor VT2 The supply voltage is supplied to the transistor collector through filter R4C4 and An oscillatory circuit is included in the drain circuit L . part of the circuit coil turns Mode by DC and temperature stabilization is ensured using resistors R1, R2 and R3. Capacity C2 eliminates negative

feedback

by alternating current. . In Fig. Figure 3 shows a circuit with a transformer connection of the circuit to the transistor collector and an autotransformer connection to the input of the next stage. Usually, in this case, an “extended” circuit setting is used (see laboratory work No. 1).

In Fig. Figure 4 shows a diagram of an amplifier cascade with FSI, made on a 265 UVZ chip
.

The microcircuit is a cascode amplifier OE - OB. Intermediate frequency amplifiers provide the receiver's main gain and adjacent channel selectivity. Their important feature is that they operate at a fixed intermediate frequency and have a large gain of the order of Using

various types

FSI, the required amplifier gain 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, for analysis you can use one generalized equivalent equivalent circuit of the amplifier (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 the parameters
and electric shock
(
, and from the input side of the next stage the conductivity
).

.
considered to be the load conductivity GN, i.e.

Analysis equivalent circuit allows you to obtain all the calculated relationships for determining the characteristics of the cascade.

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

, Where -

equivalent resonant conductivity of the circuit;

Generalized contour detuning.

From this relationship it is easy to determine the coefficient modulus

gain

and resonant gain of the RF amplifier cascade

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 given relations allow us to obtain the equation of the amplifier's resonance curve. So, with small detunes,
. From where, RF bandwidth

level 0.707 (- 3dB) is equal to

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

At ultrahigh frequencies (microwaves), the concept of power gain is more often used
- For an amplifier with a two-circuit bandpass filter, the resonant gain of the cascade is determined by the expression factor of connection between circuits, and

- coupling coefficient between circuits.

The gain (voltage) of an amplifier with any FSI when matching the filter at the input and output can be calculated using the formula
,
Here

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

- transmission coefficient of the filter in the transparency (transmission) band. In the event that the attenuation of the filter in the transparency band is known V

decibels, then Inclusion factors m And n

,
.

are calculated from the filter matching condition at the input and output The resonant characteristic of the amplifier cascade with FSI is completely determined by the transmission coefficient change curve FSI The resonant characteristic of the amplifier cascade with FSI is completely determined by the transmission coefficient change curve from frequency. Individual points of the resonance curve

are given in reference books.
The gain of the selective amplifier should 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 conductance values for the cascode circuit, for example, for the OE - OB circuit In case of use

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

The use of such amplifiers makes it possible to increase the radius of reliable radio reception; in the case of radio stations (receive-transmit devices - 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 circuits often used to increase radio sensitivity. The values of the elements used depend on specific conditions: on the frequencies (lower and upper) of the radio range, on the antenna, on the parameters of the subsequent stage, on the supply voltage, etc.

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

It is necessary to recall that the reference data for transistors provides maximum frequency parameters. 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 specified in the passport. However, for an RF amplifier connected according to the OE circuit, the maximum nameplate frequency must be reduced by at least an order of magnitude or more.

Fig.1. Circuit examples simple amplifiers high frequency (UHF) 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 in a common emitter (CE) circuit, provide relatively high gain, but their frequency properties are relatively low.

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

Figure 1 (b) shows scheme broadband amplifier high frequency (UHF) on one transistor turned on according to a common base scheme. The LC circuit is included in the collector circuit (load). 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, 1/3 of the turns are drained.

Figure 1 (c) shows another broadband circuit UHF on one transistor, included according to a common base scheme. An RF choke 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.

The values of capacitors and circuit are given for frequencies of the MF and HF ranges. For higher frequencies, for example, for the VHF range, the capacitance values should be reduced. In this case, D01 chokes can be used.

Capacitors such as KLS, KM, KD, etc.

L1 coils are chokes; for the CB range these can be coils on rings 600NN-8-K7x4x2, 300 turns of PEL 0.1 wire.

Higher gain value can be obtained by using multi-transistor circuits. These can be various circuits, for example, made on the basis of an OK-OB cascode amplifier using transistors of different structures with serial power supply. One of the variants of such a UHF scheme is shown in Fig. 1 (d).

This UHF circuit has significant gain (tens or even hundreds of times), but cascode amplifiers cannot provide significant gain at high frequencies. Such schemes are usually used at frequencies in the LW and SV ranges. However, with the use of ultra-high frequency transistors and careful design, such circuits can be successfully applied 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.

The capacitor and circuit values are given for frequencies in the CB range. For higher frequencies, such as the HF band, capacitance values and loop inductance (number of turns) must be reduced accordingly.

Capacitors such as KLS, KM, KD, etc. Coil L1 - for the CB 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 transistor UHF makes it possible to amplify radio signals. coming from antennas, in television bands - 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 is shown in Fig. 2 (a).

Fig.2.2. MV antenna amplifier circuit.

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 area low frequencies a corresponding increase in the capacities included in 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;
C 1=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. requirements for installation of HF structures: minimum lengths of connecting conductors, shielding, etc.

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

For the circuit of the given 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 response (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 the specified amplifier.

The connection diagram for 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 PEV-2 0.6. winding diameter 4 mm.

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

Using the described circuit solutions and modern high-frequency transistors (ultra-high-frequency transistors - microwave transistors), you can build an antenna amplifier for the UHF range. This amplifier can be used either 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.

Main parameters of the UHF range amplifier:

Frequency band 470-790 MHz,
Gain - 30 dB,
Noise figure -3 dB,
Input and output impedance - 75 Ohm,
Current consumption - 12 mA.

One of the features of this circuit is the supply of 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 in a circuit with a common emitter. A 3rd order high-pass filter is provided at the input of the antenna amplifier, limiting the range of operating frequencies 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 are type KD-1, the rest are 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 a diagram of connecting the antenna amplifier to the antenna socket of the TV receiver (to the UHF selector) and to a remote 12 V power source. In this case, as can be seen from the diagram, power is supplied to the circuit through coaxial cable, also used to transmit an amplified UHF radio signal from an antenna amplifier to a receiver - a VHF radio or to a TV.

Radio connection elements, Fig. 3 (b):

C5=100;
L3 - RF choke, 100 µH.

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

Setting up the amplifier comes down to setting the collector currents of the transistors and are regulated using R1 and RЗ, 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 turns of L1.

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

Fig.1. Examples of circuits of simple high-frequency (UHF) amplifiers using 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 in a common emitter (CE) circuit, provide relatively high gain, but their frequency properties are relatively low.

Figure 1 (b) shows wideband high frequency amplifier circuit (UHF) on one transistor turned on according to a common base scheme. The LC circuit is included in the collector circuit (load). 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, 1/3 of the turns are drained.

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.

Capacitors such as KLS, KM, KD, etc.

L1 coils are chokes; for the CB range these 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.

Capacitors such as KLS, KM, KD, etc. Coil L1 - for the CB 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 is shown in Fig. 2 (a).

Fig.2.2. MV antenna amplifier circuit.

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 included in 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;
C 1=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. requirements for installation of HF structures: minimum lengths of connecting conductors, shielding, etc.

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

The connection diagram for 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 PEV-2 0.6. winding diameter 4 mm.

Figure 3 shows UHF antenna amplifier circuit.

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

Main parameters of the UHF range amplifier:

Frequency band 470-790 MHz,
Gain - 30 dB,
Noise figure -3 dB,
Input and output impedance - 75 Ohm,
Current consumption - 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 are type KD-1, the rest are 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 a diagram of connecting the antenna amplifier to the antenna socket of the TV receiver (to the UHF selector) and to a remote 12 V power source. 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 to a TV.

Radio connection elements, Fig. 3 (b):

C5=100;
L3 - RF choke, 100 µH.

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

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High frequency radio frequency amplifiers. RF and Intermediate Frequency Amplifiers RF Bandpass Amplifier with AGC