VHF radio power amplifier. Scheme, description

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VHF radio power amplifier. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / RF power amplifiers

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A two-stage amplifier (Fig. 1) is designed to work with a radio station in the frequency range 144 ... 146 MHz. Its output power is up to 100 W with excitation power up to 10 W.

(click to enlarge)

In the first stage, a powerful 2P909A MOS field-effect transistor is used, which provides a power gain of 25-30 dB, a noise level of no more than 2,5 dB, and an intermodulation distortion level of no worse than -30 dB.

The second stage is made according to a bridge circuit for adding power on a balanced transistor KT9105AC, in which the influence of lead inductances is noticeably reduced due to the fact that a significant part of them is common to two transistors. It is also not required to select two transistors according to the same parameters. The small supply voltage of the amplifier and the dimensions of the KT9105AC make it possible to solve a number of problems in the design of a compact VHF radio station.

The amplifier is built on board made of foil fiberglass 2 mm thick, 180x185 mm in size (Fig. 2). Details are placed on one side of the board on the "patch", following the installation rules for this frequency range. For the 2P909A transistor, a hole with a diameter of 11 mm is drilled in the board; for the KT9105AC, a rectangular hole is cut out with dimensions of 10x25 cm. The power supply voltage of the amplifier is supplied through the conductors in the screen, which is soldered along the entire length of the board.

Amplifier coils are frameless, wound with silver-plated copper wire. Coils L1, L2 contain 3 turns of wire with a diameter of 0,8 mm. The distance between the turns is 2,5 mm, the inner diameter is 3 mm, the inductance is 0,3 μH. Coils L4, L5, L10, L11 have 2 turns of wire 0,8 mm each, inner diameter 10 mm, distance between turns 2,5 ... 3,0 mm, inductance 0,2 μH. Coil L12 contains 4 turns of wire with a diameter of 1,5 - 1,8 mm. The inner diameter of the coil is 10 mm, the inductance is 0,3 μH.

Inductor L3 - unified HF inductor D2 with an inductance of 1 μH. Chokes L6, L7 contain 10 turns of wire PEV-2 0,33. They are frameless, their inner diameter is 2,5 mm, the distance between the turns is 1-1,5 mm, the inductance is 0,4 μH.

Chokes L8, L9 have 10 turns of wire PEV-2 0,8. Frameless winding, inner diameter 6 mm, inductance 0,6 μH.

Capacitors C1 and C2 - KPK-MN, KPKMN. The remaining trimmer capacitors - with an air dielectric KPV, KPVM.

Permanent capacitors - leadless of any type.

Zener diode VD1 of any type for 15 V (0,5 W). Diodes VD2, VD3 2D212 (A or B) or KD212A, 2D215V, KD226A, KD212V, KD208A.

It is very convenient to use CHIP components. Produced CHIP - inductances, have an inductance from 0,1 to 1000 μH, their dimensions are 4,5x3,2 mm; 3,2x2,5 mm. Leadless resistors and capacitors (have a tolerance of 2%) are manufactured in flat packages. The special design of the terminals improves the assembly and the quality of the connections of the parts.

The minimum distance from the body of the transistor 2P909A to the soldering point of the leads is 1,0-1,5 mm. Soldering temperature no more than +250° С. Soldering time no more than 3 s. For the KT9105AC transistor, the bending of the leads is allowed no closer than 3 mm from the case, and soldering is no closer than 1 mm at a temperature of not more than +265 ° C for 4 s. The transistor can be mounted by soldering the flange to the heat sink at a temperature of not more than + 150 ° C for 2 minutes.

The heat sink of the amplifier has a ribbed surface 50 mm high, its total surface area is not less than 300 sq.cm. Transistors must have reliable thermal contact with the heat sink. In the first stage, transistors 2P909B, 2P909V or foreign IRF510, IRF511, IRF520, IRF530, IRF531 can be used.

Instead of VT2, it is permissible to use two bipolar transistors, selecting them according to the appropriate parameters. In this case, the heat sink area is increased by about 1,7 times.

When setting up the amplifier, the quiescent currents are set to 50 ... 70 mA in the first stage, in the second - by selecting R7 and R8, the same quiescent currents are within 300 ... 320 mA.

Then the circuits in the amplifier are tuned to a frequency of 144 ... 146 MHz by selecting the number of turns of the coils and shifting them. After that, the power of the amplifier is measured.

When working on a matched load, the supply voltage in the power amplifier should not exceed 35 ... 40 V.

Author: Gennady Osipov (RV3AK), Moscow; Publication: N. Bolshakov, rf.atnn.ru

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