How automatic gain control (AGC) circuit actually actually works?

Question

How automatic gain control (AGC) circuit actually actually works?

Answer 1

The audio signal is put through a low pass filter with a time constant of about 1 minute (super low pass) or at least many seconds.  That's a huge RC - a large electrolytic capacitor; a large resistor.  That almost-DC signal output from the filter is a representation of the long-term level of the audio.  If some fading is present at the input of the device (radio), then over the long-term the audio average drop and that 'DC' signal will decrease.   Likewise if the input level increases over the long-term, then the opposite happens.

The 'DC' signal is fed to a VCA -- a voltage controlled amplfier.  VCAs are available in a wide variety of configurations.  One common configuration is to use a JFET in the 'triode' mode.  That is, as the gate-to-source voltage changes (from that 'DC' signal), the drain-to-source resistance changes.  This change in drain-to-source resistance is used as a feedback resistor on an op-amp to change the gain.

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