Automatic gain control circuit

摘要

A circuit for controlling the gain of an amplifier to prevent signal clipping even if the power supply voltage drops. The AGC amplifier has one input electrode connected to the output of the main amplifying circuit and a second input electrode connected by a non-linear circuit to one terminal of the power supply to vary the bias on this transistor with changes in the power supply voltage so as to stabilize the operation of the transistor in spite of changes in the amplitude of the audio signal due to the changes in the power supply voltage. The transistor furnishes an AGC signal to control a variable impedance across the input of the amplifier to be controlled. The variation in this impedance reduces the gain of the controlled amplifier when amplifying high signal levels.