Power amplifier for radio stations of the 3rd category. Scheme, description

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Power amplifier for radio stations of the 3rd category. 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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Not only for a radio station of the 3rd category, but also for the operation of QRP radio stations of the 1st and 2nd categories, the transistor power amplifier circuit for CW and SSB operation, shown in Fig. 1, may be of interest.

Ris.1

Transistor power amplifiers have a very low (units or tens of ohms) output impedance, which creates certain difficulties in making their connection circuits with the antenna.

In particular, in such amplifiers it is unacceptable to use long (up to 10 cm) wires, which are necessary to switch the resonant communication circuits with the antenna for each range. Therefore, high-frequency transistor power amplifiers are often made with broadband antenna-coupling transformers, and these transformers are quite critical in their characteristics.

The scheme is single-range. Switched circuits can be used to operate on 40, 80 and 160 meters, but it is more rational to make a single-band amplifier.

The device is designed to operate on exciters with an output power of about 1 watt. The exciter load, which ensures stable (without signs of self-excitation on all ranges) operation of the amplifier, is resistor R1. A powerful field-effect transistor VT2 is used as a linear power amplifier.

To ensure good gain linearity, the transistor VT2 operates at a high value of the quiescent current - in the absence of an excitation voltage, by selecting the resistance of the resistor R4, a constant current in the drain circuit of VT2 is set to about 0,3 A. A single-tone signal with a voltage of 1 V at the RF Input connector will increase the DC component of the current through VT2 at 0,7 A with the allocation of the C5 L3C6 circuit to the antenna with a power of 10 watts.

The scheme provides for switching reception - transmission. It is carried out from an external control circuit, which is closed to the housing when switching to gear. In this case, the high-frequency relay K1 is activated, the contacts of which disconnect the antenna from the receiver and connect it to the output of the power amplifier.

In the receive mode, the control circuit is open and a positive voltage is applied to the base of the transistor VT1, which opens it. The voltage at the collector VT1 becomes close to zero, and VT2 closes.

Relay K1 type RPV2/7, passport RS4.521.952. Inductors L1 and L2 type D-1 (at 1A) with an inductance of 30 and 10 μH, respectively.

The circuit data at the output of the amplifier are given in the table.

Range, m	C5,pf	С6,pf	L3, coils
160	6200	3000	20
80	3000	1500	15
40	1600	820	10
20	750	420	7
15	470	300	5
10	300	200	4

Coils L3 - single-layer, frame diameter 15 mm (wire PEV-2 1,5). The winding length is selected for each band until the maximum output power is obtained at the center frequency.

Authors: B.Stepanov, Y.Lapovok, G.Lyapin; Publication: N. Bolshakov, rf.atnn.ru

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