High-frequency amplifiers on microcircuits. High-frequency RF power amplifier generators on striplines

The main area of ​​application is as part of vacuum processing equipment to ensure stable and controlled processes for applying functional coatings, plasma cleaning and etching in vacuum deposition systems.

HF GENERATOR “IVE-171RFS”

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The RF (RF) generator "IVE-171RFS" is single-channel - it has an output high-frequency voltage with a frequency of 13.56 MHz and is designed to supply its active load with a resistance of 50 Ohms or to a "capacitive high-impedance load" when working in conjunction with an automatic matching device "ASU- 171S". The RF generator has a galvanically opto-isolated external control interface “RS-485”, oriented for operation as part of automated equipment.

BASIC TECHNICAL DATA

Output active RF (RF) power, adjustable*, W.....from 30 to 600

Output amplitude RF (RF) voltage, adjustable*, V.....from 10 to 250

Instability of output RF (RF) power, %, no more.....3

Instability of output RF (RF) voltage, %, no more.....2

Output voltage frequency, MHz.....13.56

Maximum amplitude voltage of RF protection, V.....280

Maximum peak current of RF arc protection, A.....6

Limit of output reactive amplitude RF power, VA.....1680

Efficiency (RDC/RAS // RHF/RDC // RHF/RAS), not less than.....0.85//0.55//0.467

Electrical power consumption, W.....1300

Supply voltage.....220V -15% +10%, 48~62 Hz

Weight, kg.....15

Overall dimensions, mm.....224 x 133 x 417

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* on a resistive equivalent load with a resistance of 50 Ohms via cable IVE4.171.030

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The HF generator “IVE-171RFS” is a secondary power supply with a transformerless network input, operating at conversion frequencies of 45¸55 kHz and 13.56 MHz. The block is based on two assemblies of transistor converter modules, powered by: the first - variable mains voltage from a single-phase noise-suppressing network filter, the second - by constant regulated power intra-unit voltage received from the first module. Both are controlled via a control and signal interface module. Thus, the conversion of the mains supply voltage into a high-frequency output voltage is carried out sequentially in two stages using two modules: the first converter module and the second high-frequency power amplifier module. From the output controlled DC voltage 0V÷+60V of the first low-voltage converter module, with a power of 1.2 kW, the high-frequency power amplifier module is powered. Its RF output voltage with an amplitude of 0V÷280V and reactive power up to 1680VA, goes to the output connector of the RF generator. To consume almost sinusoidal current from the supply network, the converter module circuitry and functionally performs power factor correction. The formation of all operating algorithms, processing and generation of control signals of data signals is carried out in the control and signal interface module. RF voltage with a frequency of 13.56 MHz is generated in a high-frequency power amplifier module, which includes a quartz master oscillator. Then it goes to the RF pre-amplifier, and then to the final RF power amplifier, located in the same module. From the output of the final RF power amplifier, the RF voltage is supplied to the input of the “RF measurement” unit, also located in the high-frequency power amplifier module, and from its output to the output connector of the unit. The high-frequency power amplifier module, when operating on a nominal resistive load of 50 Ohms, provides a maximum of 600 W of active power.

In addition, the high-frequency power amplifier module has a thermal protection unit that turns off the RF voltage when the power RF transistors or protective-limiting circuits overheat due to operation at unmatched modes (loads) or when operating at an ambient temperature of more than +35°C. The signals of the output RF voltage, RF current and active RF power of the RF generator, converted in type and level in the “RF measurement” unit, are sent for further processing to the control and signal interface module and to the RF protection unit. The RF protection unit calculates the reactive output RF power, the ratio of active to reactive RF power and generates signals to prohibit the operation of the final RF power amplifier when the specified levels of the output amplitude RF voltage and RF current are exceeded, thereby ensuring its protection. The RF generator has a 3.5-digit digital indication of output and set parameters: power, voltage, current, and their regulation from the front panel of the unit, as well as LED indication of all operating modes and, accordingly, their selection using buttons located on the front panel block. These indication and control elements constructively form an indication and control module. The RF generator is equipped with a fan control unit, which maintains constant thermal conditions of the converter module and high-frequency power amplifier module and increases the fan's operating life. The RF generator contains standby and service power supply modules that respectively generate standby voltage ±5V, which is necessary for the operation of the mains filter and the control and signal interface module, and service voltage +24V, which is necessary to power the fan control unit and the high-frequency power amplifier module.

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The output impedance of the high-frequency power amplifier module is matched with the load impedance by the automatic matching device "ASU-171S", capable of operating in both automatic matching mode and manual control mode from a manual control panel.

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It is possible to develop and manufacture a high-frequency generator with different output characteristics according to your technical specifications.

AUTOMATIC MATCHING DEVICE “ASU-171S”

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The automatic matching device "ASU-171S" is a T-shaped "CCL-circuit" tunable by means of two variable vacuum RF capacitors, which allows you to match the load impedance with the output impedance of the RF generator. The RF RF voltage coming from the generator to the “RF INPUT” connector, passing through the “RF signal” meter installed in the ACS, enters the matching “CCL circuit” and, converted in level, is output to the output coaxial connection “RF OUT”. Having passed through the mixer assembly, it also receives a constant voltage supplied to the “DC INPUT” input connector, the maximum value of which should not exceed 1000V, and its current should not exceed 2A. Variable capacitors have an electric drive controlled by a control and interface module, which, based on the operating algorithm embedded in it and, receiving signals about the values ​​of the amplitude RF voltage, RF current and active RF power from the RF signal meter, generates commands for electric drive in such a way that the load impedance “reduced” to the ACS input approaches 50 Ohms, and the double value of the ratio of effective active RF power to reactive RF power tends to unity.

The control and interface module, in addition to generating power control signals for the electric drive of variable RF capacitors, provides amplification and conversion of signals about the magnitudes of amplitude RF voltage, RF current and active RF power from the RF signal meter. Having processed them, it outputs the ratios of amplitude RF voltage to RF current and effective active RF power to reactive RF power in analog form to the “CONTROL” connector and in the form of a serial digital code to the “RS-485” connector of the external control interface. In addition, the control and interface module provides conversion and interface of control signals and ACS modes from the RS-485 interface and the manual control panel, as well as conversion of the mains supply voltage to constant voltages±24V for powering the electric drive of variable HF capacitors.

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We continue the conversation about the direct amplification transistor receiver, which began at the seventh workshop. By then connecting the detector receiver to a single-stage low-frequency amplifier, you thereby turned them into a 0-V-1 receiver. Then I assembled a single-transistor reflex receiver, and at the previous workshop I added a two-stage low-frequency amplifier to it - the result was a 1-V-3 receiver. Now try adding a high frequency (HF) modulated preamp stage to it to make it a 2-V-3 receiver. The sensitivity in this case will be sufficient to receive not only local, but also distant broadcasting stations on the magnetic antenna.

What is required for such a single-stage RF amplifier? Basically - a low-power high-frequency transistor of any of the P401...P403, P416, P422, GT308 series, as long as it is in good working order, several capacitors, a resistor and a ferrite ring of grade 600NN with an outer diameter of 8...10 mm. The coefficient h21E of the transistor can be in the range of 50...100. You should not use a transistor with a large static current transfer coefficient - an experienced amplifier will be prone to self-excitation.

Schematic diagram amplifier is shown in Fig. 56. The amplifier itself is formed only by a transistor V1 and resistors R1, R2. Resistor R2 acts as a load, and the base resistor R1 determines the operating mode of the transistor. The collector load of the transistor can be a high-frequency choke - the same as in a reflex receiver.

Custom contour L1 C1 and communication coil L2 refer to the input circuit, capacitor C2- dividing. This part is an exact repetition of the input part of the receiver you have already tested. Capacitor Immediately, resistor R, diode V2, phones B1 s The blocking capacitor Sbl forms a detector circuit necessary for testing the amplifier.

How does such an amplifier work? Fundamentally the same as a single-stage low-frequency amplifier. It only amplifies not audio frequency oscillations, like that amplifier, but modulated high frequency oscillations coming to it from the coupling coil L2. The high-frequency signal, amplified by the transistor, is allocated to the load resistor R2 (or other collector load) and can be fed to the input of a second stage for additional amplification or to a detector to convert it into a low-frequency signal.

Mount the amplifier parts on a temporary (cardboard) board, as shown on the right in Fig. 56. Move here and connect the parts of the input circuit (L1C1) and the communication coil (L2) of the receiver to the amplifier. Don't forget to include an isolating capacitor in the coupling coil circuit C2. Connect the battery voltage 9 V and, choosing a base resistor R1, install collector current transistor within 0.8...1.2 mA. Don’t forget: the resistance of the base resistor should be greater, the greater the static current transfer coefficient of the transistor (the value of this resistor indicated in the diagram corresponds to the coefficient h21E transistor about 50).

Now mount a detector circuit on a separate small cardboard, connecting in series the phones B1 with a blocking capacitor Sbl with a capacity of 2200..3300 pF, a point diode V2 any series and separator nyu capacitor Immediately with a capacity of 3300...6800 pF, Resistor resistance R maybe 4.7...6.8 kOhm. Connect this circuit between the collector and emitter of the transistor, that is, to the output of the amplifier, and connect an outdoor or indoor antenna and, of course, grounding to the input circuit L1C1. When tuning the input circuit to the wave of a local radio station, its high-frequency signal will be amplified by the transistor VI, detected by diode V2 and converted by phones IN 1 into sound. Resistor R in this circuit is necessary for normal operation of the detector. Without it, phones will sound quieter and distorted.

For the next experiment with an RF amplifier, a high-frequency step-down transformer is needed (Fig. 57). Wind it on a ring of 600NN grade ferrite (the same as the core of the high-frequency choke of the reflex stage of the receiver). Its primary winding L3 should contain 180..200 turns of wire PEV or PEL 0.1...0.12, and the secondary L 4 60...80 turns of the same wire.

Connect winding L3 of the high-frequency transformer to the collector circuit of the transistor instead of the load resistor, and to its winding L4 connect the same detector circuit as in the previous experiment, but without the coupling capacitor and resistor, which are not needed now. How does it sound now? phones? Louder. This is explained by better matching of the output impedance of the amplifier and the input impedance of the detector target than in the first experiment.

And now, using the diagram shown in Fig. 58, connect this single-stage amplifier to the input of the 1-V-3 reflex receiver transistor. The RF receiver amplifier became two-stage. The connecting element between the cascades was the coil L4 high-frequency transformer included in the base circuit of transistor V 2 (in the receiver 1-V-Z the transistor W1 was used) instead of the communication coil (there was L2) with the former input configurable circuit. Now an external antenna and grounding are not needed - reception is carried out using the magnetic antenna W1. whose role: is performed by a ferrite rod with a coil located on it L1 input configurable circuit L1 C1.

So, together with a two-stage low-frequency amplifier, a four-transistor direct amplification receiver 2-U-W was trained. The receiver may be self-exciting. This is because, firstly, it is reflexive, and reflexive receivers are generally prone to self-excitation, and secondly, they are conductors connecting the experienced amplifier stage with reflex cascade, long. If the new stage, together with the magnetic antenna, is mounted compactly on the same receiver board, making the circuits as short as possible, there will be fewer reasons for self-excitation. This is also facilitated by the decoupling filter cell. R2 C3 in the negative power circuit of the first transistor of the RF amplifier, which eliminates the connection between the stages through a common lithium source and thereby prevents self-excitation of the high-frequency path of the receiver.

But the second stage of the RF amplifier may be the same as the first, that is, not reflexive, and the connection between them may not be a transformer. Diagram possible option amplifier is shown in Fig. 59. Here the load of the transistor V1 the first stage, as in the first experiment of this workshop (see Fig. 56), is resistor R2; The high-frequency signal voltage created across it through a capacitor NW supplied to the base of the transistor V2 the second cascade, exactly the same as the first. The signal, additionally amplified by the transistor of the second stage, is removed from its load resistor R4 ( the same; like R 2) and through capacitor C 4 (such as NW) goes to the detector on diode V 3, is detected by it, and the low frequency oscillations created across its load resistor R5, are fed to the input of the bass amplifier.

In this version, the second cascade and detector are like an unfolded reflex cascade of the previous version. But the transistor only amplifies high-frequency oscillations. And if you connect it to a two-stage low-frequency amplifier, you get a direct amplification receiver 2- V-2. The amplification of the low-frequency signal will decrease somewhat, telephones or the loudspeaker head at the output of such a receiver will sound a little quieter, but the danger of self-excitation of its high-frequency path will be reduced. This loss can be partially compensated by increasing the voltage of the low-frequency signal at the output of the detector by including a second diode in the detector cascade (shown in dashed lines in Fig. 59 V4), as you did in one of the experiments in the seventh workshop (see Fig. 50), or use a transistor in the detector cascade.

Try to experiment with low-frequency amplifier options, compare the quality of their work and draw appropriate conclusions for the future.

One more tip. When experimenting with one or another version of the receiver, draw and remember its complete circuit diagram. For what? A radio amateur, even a beginner, must draw diagrams of such devices from memory. The circuit diagram will also help you better understand the operation of the receiver as a whole and its parts, and will make it easier to find faults in it.

Literature: Borisov V.G. Workshop for a beginner radio amateur. 2nd ed., revised. and additional - M.: DOSAAF, 1984. 144 p., ill. 55k.

Current consumption - 46 mA. The bias voltage V bjas determines the output power level (gain) of the amplifier

Fig. 33.11. Internal structure and pinout of TSH690, TSH691 microcircuits

Rice. 33.12. Typical inclusion of TSH690, TSH691 microcircuits as an amplifier in the frequency band 300-7000 MHz

and can be adjusted within 0-5.5 (6.0) V. The transmission coefficient of the TSH690 (TSH691) microcircuit at a bias voltage V bias = 2.7 V and a load resistance of 50 Ohms in a frequency band up to 450 MHz is 23 (43) dB, up to 900(950) MHz - 17(23) dB.

Practical inclusion of TSH690, TSH691 microcircuits is shown in Fig. 33.12. Recommended element values: C1=C5=100-1000 pF; C2=C4=1000 pF; C3=0.01 µF; L1 150 nH; L2 56 nH for frequencies not exceeding 450 MHz and 10 nH for frequencies up to 900 MHz. Resistor R1 can be used to regulate the output power level (can be used for a system automatic adjustment output power).

The broadband INA50311 (Fig. 33.13), manufactured by Hewlett Packard, is intended for use in mobile communications equipment, as well as in household electronic equipment, for example, as antenna amplifier or radio frequency amplifier. The operating range of the amplifier is 50-2500 MHz. Supply voltage - 5 V with current consumption up to 17 mA. Average gain

Rice. 33.13. internal structure microcircuits ΙΝΑ50311

10 dB. The maximum signal power supplied to the input at a frequency of 900 MHz is no more than 10 mW. Noise figure 3.4 dB.

A typical connection of the ΙΝΑ50311 microcircuit when powered by a 78LO05 voltage stabilizer is shown in Fig. 33.14.

Rice. 33.14. broadband amplifier on the INA50311 chip

Shustov M. A., Circuitry. 500 devices on analog chips. - St. Petersburg: Science and Technology, 2013. -352 p.

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 allows you 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 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.

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, when using transistors ultra high frequency and careful execution, 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 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.

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(oscillating 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 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.

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.