VHF power amplifier. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / RF power amplifiers

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The idea to use the field-effect transistor KP904A in the power amplifier of the 2 m band arose involuntarily - while working in the "tropo" the KT931A transistor failed, and there was nothing to replace it with. Then the choice fell on KP904A (according to reference data, it is operational up to a frequency of 400 MHz).

The amplifier on this transistor is not critical to the quality of the power supply (in my case, it is powered by an unstabilized voltage of +55 V with a capacitance of the output capacitor of the power supply of 10000 uF), does not require special measures to stabilize the quiescent current of the transistor and has a very simple circuit (Fig. 1) . With an input power of 4 ... 5 W, the output power is 20 ... 25 W at a load of 75 ohms.

Ris.1

To obtain a greater output power (about 40 W), two similar stages can be switched on in series, supplying the pre-terminal stage with a voltage of +28 V, and the final stage - +50 V (Fig. 2).

Ris.2

The amplifier is assembled on a ribbed radiator with dimensions of 210x130 mm, the height of the ribs is 25 mm. The mounting of the radioelements is made on "spots" cut out on the board from the foil textolite.

During the installation process, special attention must be paid to the quality of the connection between the source of the transistor and the common wire ("case"). In my case, the source is soldered to a silver-plated plate, which, in turn, is soldered around the transistor hole on the board.

Resistor R1 is soldered directly to the terminals of the transistor VT1 and protects the transistor from failure if the bias circuit is open.

The winding data of the inductors are shown in the table.

Capacitor C 10 is of the SGM type, C6 is electrolytic, for an operating voltage of 100 V. The rest of the capacitors are of the KM type.

Setting up this amplifier is very simple. An equivalent load with a resistance of 75 (50) Ohm is connected to its output, and by rotating the slider of the resistor R3 (Fig. 1), the quiescent current of the transistor VT1 is set to about 200 mA. Then, a signal with a frequency of 145 MHz and a power of about 1 W is applied to the input of the amplifier (for example, from a "handic") and, by adjusting the capacitors C2, C3, they achieve the maximum current through the transistor VT1, and with the help of capacitors SI, C12 - the maximum voltage on the equivalent loads.

After the described pre-setting of the amplifier, a 5 W signal is applied to its input and the capacitors C11, C12 are again adjusted to the maximum output power. The amplifier circuit does not have protection against short circuit and from exceeding SWR, therefore, work on random antennas should be avoided. This recommendation is especially relevant for the two-stage version of the amplifier.

Author: A. Zhuk (EW6FS), Postava, Radio amateur; Publication: N. Bolshakov, rf.atnn.ru

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