In the AGC circuit of Fig 1, a 4-quadrant analog multiplier (IC_{1}), an amplifier stage (IC_{2}), an active, full-wave rectifier (D_{1}, D_{2}, R_{4}-R_{7}, and IC_{3}), and an integrator (IC_{4}) accomplish automatic gain control of V_{IN}’s amplitude variations in the audio-frequency range.
Figure 1. | Analog multiplier IC_{1} combines V_{IN} with a feedback signal V_{Y }to achieve automatic gain control. |
The multiplier’s output is –V_{|N}V_{Y}/10, where V_{Y} is a negative voltage generated by the integrator IC_{4}. Together, the integrator and the rectifier extract the dc component (V_{Y}) of V_{OUT} for use as a feedback signal to the multiplier. The integrator sums signal current from the rectifier and control current from potentiometer R_{9}, which lets you adjust V_{OUT}’s signal level.
Circuit analysis yields the frequency-response equation
or, in the time domain,
In both equations, K_{1} is the gain of amplifier IC_{2}, A is the peak amplitude of V_{IN}, and R is the resistance between the integrator input and the rectifier output. (For this circuit, R equals R_{6} in parallel with R_{7}.)
This AGC circuit is suitable for controlling the long-term variations of amplitude within a limited range. It doesn’t respond uniformly over a wide dynamic range, however, because the time response is inversely proportional to input-signal amplitude.