| This circuit is an AGC system for
audio-frequency signals. AGC systems usually consist of three parts: an amplifier,
rectifier, and controlled impedance. In this circuit the functions of an amplifier and a
rectifier are performed by a single op amp. This makes the system simple and cheap. The
rectifier is made with the output push-pull cascade of the op amp and Rs, RL, and CB. The
transistor Q1 and D1 are used as a voltage-controlled resistance (Z). The input signal is
(Z+R1)/Z times, diminished by the voltage divider and 1+R2/R1 times, amplified by the op
amp. C2 eliminates influence of dc bias voltage. R3 protects Q1 and D1 from excessive
current. |