Broadband power amplifier. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / RF power amplifiers

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Among shortwaves, low power operation - QRP - is gaining more and more popularity. Particularly attractive is that the hardware for QRP can be made entirely on transistors and powered from a low-voltage source.

The broadband power amplifier, the circuit of which is shown in the figure, provides linear amplification of SSB and CW signals in all KB bands. The power supplied to the terminal stage at a supply voltage of 12 V does not exceed 10 W, which corresponds to the international definition of QRP equipment.

(click to enlarge)

The signal from the KB exciter (transceiver) is fed through an emitter follower (transistor VTI) to a preamplifier operating in mode A (transistor VT2). The input impedance of the next stage is about 10 ohms, therefore, to match it with the preamplifier, a broadband transformer T1 with a transformation ratio of 4:1 (in terms of resistance) was used. Capacitors C29 and C13 equalize the frequency response of this stage.

The push-pull output stage (VT4, VT5) operates in AB mode. It is connected to the load through a broadband three-winding transformer T2 with a transformation ratio of 1:1. Such a transformer makes it possible to exclude the saturation of the magnetic circuit with the constant component of the collector current. A constant quiescent current of the output transistors is maintained by a stabilizer on the transistor VT3. Diodes VD1 and VD2, which set the bias voltage of transistors VT4 and VT5, are in thermal contact with their heat sinks.

When setting up the amplifier, the output stage is first turned off, the secondary winding of the transformer T1 is loaded with two resistors connected in series with a resistance of 8,6 ohms, and the point of their connection is connected to a common wire. By selecting the resistor R2, the quiescent current of the transistor VT2 is set (~ 100mA). Then a high-frequency voltage is applied to the input of the amplifier. In a normally operating pre-amplifier with an input voltage of 220 mV, the signal limitation in the collector circuit of the transistor VT2 should be at the level of 9 V, and the signal amplitude at each of the terminals of the secondary winding of the transformer T1 (with respect to the common wire) should be about 1,1 V.

Then the emitter of transistor VT3 is connected to a common wire through a 17 Ohm resistor and a tuned resistor R6 is set to 0.75 V at the output of the stabilizer. resistor R50 set the total quiescent current of transistors VT5, VT6 equal to 4 mA.

The output voltage should be limited to 22 V, and the current consumed by the final stage should be within 0.7 ... 0,8 A.

One of the criteria for stable (without self-excitation) operation of a power amplifier is a smooth, without jumps, increase in current. consumed by the output stage, with an increase in the exciting RF voltage.

Transistors SF137E and SF126E can be replaced respectively by KT312V and KT815B, and diodes SAY12 - by KD503A.

Literature

1. Doerenzg W. Eine einfache 10-W-QRP-Endstufe fur Kurzwelle.- Funkamateur, 1984. No. 2. S. 89-91.

Publication: N. Bolshakov, rf.atnn.ru

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