Power amplifier control unit. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Power amplifier control unit. 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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On amateur KB radio stations, CW and SSB signal power amplifiers are widely used, assembled according to a grounded grid circuit on triodes, as well as tetrodes or pentodes in a triode connection.

One of the possible options for the control unit of such an amplifier is shown in the figure. The amplifier is assembled on a VL1 lamp (high-frequency circuits are not shown for simplicity, L1 is an anode choke, L2 is a cathode choke). When working as a telegraph, the triode operates in class B (zero bias voltage on the grid and almost zero quiescent current), and when the SSB signal is amplified, a certain voltage is applied to the lamp cathode, slightly opening the lamp. The quiescent current in this case is set by a tuning resistor R2. The operating mode of the amplifier is selected by the switch SA1. Resistor R5 protects the transistor VT2 from failure due to transients in the control node. The grid current of the VL1 lamp is measured by the RA1 device, the anode current - by the RA2 device. When high voltage is applied to the VL1 lamp, the arrow of the PA1 device deviates slightly to the left, which indicates the normal operation of the control unit.

Power amplifier control unit

From reception to transmission, the power amplifier is switched by switch SA2 (these can be VOX relay contacts, a pedal, etc.). When the contacts of this switch are closed, the relays K2 and K3 are activated, which, with their contacts (not shown in the figure), switch the high-frequency circuits at the input and output of the power amplifier. Relay K1 operates with some delay, determined by the charging time of the capacitor C1 through the decoupling diode VD3 and resistor R6. As soon as this relay is activated, a bias voltage will be applied to the base of transistor VT2 through resistor R1 (when the signal is amplified). When working as a telegraph, the base circuit of this transistor is connected to a common wire through the contacts of the SA1 switch, and the operating mode of the VL1 lamp does not change in this case. At the same time, through contacts K1.2, the supply voltage is supplied to the LED HL1. indicating the operation of the power amplifier in transmit mode. The delay in the operation of relay K1 with respect to relays K2 and K3 eliminates the appearance of an RF signal at the output of the amplifier until the antenna is connected to it.

Quite high requirements are imposed on the regulating transistor VT2. Its maximum collector current must be at least the possible peak current of the VL1 lamp, and the allowable collector-emitter voltage must be at least the peak value of the excitation voltage at the lamp cathode. Radio frequency currents do not flow through transistor VT2, so it can be low-frequency.

The voltage drop across resistor R4 is used to create the automix voltage. The value of this resistor should be chosen based on the output power and the type of lamp used in the output stage.

Requirements for the transistor VT2 are specified in the text. As a transistor VT1, you can use transistors of the KT605, KT608 series. Diodes VD1-VD4 - any of the D226 series.

Author: Dierking H.; Publication: N. Bolshakov, rf.atnn.ru

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