Editing Automatic gain control (section)

{{short description|Electronic circuit to automatically adjust signal strength}}
[[File:Vactrol AGC (Yushin).PNG| right | thumb | Schematic of an AGC used in the analog telephone network; the feedback from output level to gain is effected via a [[Vactrol]] resistive opto-isolator.]]

'''Automatic gain control''' ('''AGC''') is a closed-loop [[feedback]] regulating circuit in an [[amplifier]] or chain of amplifiers, the purpose of which is to maintain a suitable signal amplitude at its output, despite variation of the signal amplitude at the input. The average or peak output signal level is used to dynamically adjust the [[gain (electronics)|gain]] of the amplifiers, enabling the circuit to work satisfactorily with a greater range of input signal levels. It is used in most [[radio receiver]]s to equalize the average  volume ([[loudness]]) of different radio stations due to differences in received [[signal strength]],   as well as variations in a single station's radio signal due to  [[fading]].  Without AGC the sound emitted from an [[amplitude modulation|AM]] [[radio]] receiver would vary to an extreme extent from a weak to a strong signal; the AGC effectively reduces the volume if the signal is strong and raises it when it is weaker.  In a typical receiver the AGC feedback control signal is usually taken from the [[detector (radio)|detector]] stage and applied to control the gain of the IF or RF amplifier stages.