Amplifier. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / RF power amplifiers

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The power amplifier was designed specifically for the operation of the QRP on the low bands. Output power at a load of 50 ohms - 10 watts. Thanks to the introduced negative feedback (NFB), its amplitude-frequency response (AFC) is uniform up to 21 MHz, and at a frequency of 30 MHz there is a decline of -3 dB. A feature of the circuit is the absence of matching transformers between stages. Optimum load resistance (RH) - 50 ... 150 Ohm. Smaller values ​​of RH lead to a violation of the thermal regime of the output transistors. At an output power of 10 W, the intermodulation distortion coefficient is -43 dB, at 5 W -52 dB. The sensitivity of the amplifier is 50 mV, and it can be recommended for use with such well-known designs as "single-board path", "Radio-76", etc.

The circuit diagram of the amplifier is shown in Fig. 1, and Fig. 2 shows the power supply circuit.

Fig.1 (click to enlarge)

The input voltage is amplified by the transistor VT1. R6, C2 connected in series in the feedback circuit equalize the frequency response of the input stage. Transistors VT2 and VT3 - emitter followers - match the input amplifier with a low input impedance of the final stage. An intermediate amplifier operating in class "A" is assembled on the VT4 transistor. Diodes VD1 ... VD4 sets the initial current of the output stage on transistors VT5 and VT6.

The final stage has a low output impedance, therefore, an RF transformer with a transformation ratio of 1: 2 in voltage is installed to match the load. Diodes VD5 and VD6 are designed to protect the output transistors from overvoltage. The final amplifier is covered by an OOS circuit consisting of C15, R12 and R11. The depth of the OOS is approximately 10 dB.

For the normal operation of the power amplifier, it is very important that the supply voltage VT4 be exactly half the supply voltage (Upit) of the output stage transistors VT5, VT6.

Switching the "reception-transmission" mode is carried out by switching the supply voltage of the input stage VT1, VT2 and bias on the transistor VT4. In this scheme, telegraph manipulation is easily implemented. It is made on the transistor VT7.

In SSB mode, the VT7 transistor is open, and + 12V is supplied to the input stage. In CW mode, VT7 opens only when the telegraph key is pressed. The leading edge of the telegraph message is set (determined) by the chain R26, C22, and the rear edge - C22, R25.

The assembly of the amplifier can be carried out by hinged mounting "in a ruler" or on a printed circuit board made of double-sided foil fiberglass, one of the sides of which is used as a screen (from the side where the elements are installed). It is necessary to ensure that the mounting capacitance of the base circuits of transistors VT3, VT5 and VT6 is minimal. Transistors VT3, VT4. VT5 and VT6 are installed on a common radiator.

Setting up the amplifier comes down, first of all, to checking and setting the modes for direct current. First, a source of + 12V and + 12V TX is connected. The selection of R2 sets the voltage + 6V on the collector of the transistor VT1. With serviceable transistors VT2 and VT3, the voltage at the VT3 emitter will be + 4,6V. Then the + 25V source is connected and by selecting the resistor R17 the quiescent current of the transistor VT4 is set within 70 ... 80 mA. Only after the settings have been made, a voltage of + 50V is applied, and by selecting the type and number of diodes VD1 ... VD4, the quiescent current of the output transistors VT5 and VT6 is set to 40 - 60 mA.

When an input signal is applied, the current of the output transistors increases to 300 - 400 mA. The RF voltage at a load of 75 ohms at a frequency of 1,9 MHz is at least 28 Veff. The diagram (Fig. 1) shows the RF voltage levels on individual elements of the circuit.

The power amplifier power supply (Fig. 2) is assembled on integrated circuits KREN12. The output voltage is adjusted by resistors R3, R4.

Fig. 2 QRP amplifier power supply (click to enlarge)

Missing elements can be replaced by VT 1 - KT325, KT399, KT316, etc.
VT2 - KT610, KT630, KT608,
VT3-KT610,
VT4 - KT904,
VT5-KT922,
VT6 - KT933.
Diodes VD1 ... VD4 - any silicon.

Capacitor C18 is made up of several ceramic capacitors of different capacities.

Transformer T1 is wound with double twisted wire PEV-2 - 0,51 on a ferrite toroidal core 600NN KZ 2x16x6 mm. The twist pitch is one turn per 2 cm. Contains 10 ... 15 turns. It is noticed that winding the transformer on a ferrite cup improves the frequency response of the amplifier.

The amplifier has good repeatability and high operational reliability.

Author: M. Roslanov, UA4UDF, Saransk; Publication: N. Bolshakov, rf.atnn.ru

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