Audio Level Control Device: compressor circuit based on LED and photodiode  

Automatic gain Control (AGC) Circuits:

Digital Automatic Gain Control (AGC):

High Frequency AGC has Digital Control: 05/21/98 EDN-Design Ideas 

Wideband AGC Amplifier As a Differential Amplifier: National Semiconductor Application Notes,10-Apr-2000 

Wideband AGC has 60 DB Dynamic Input Range: 10/10/96 EDN-Design Ideas