Agc circuit to maintain amplification at a fixed level between speech bursts

Claim 1

An automatic gain control circuit for a signal amplifier comprising a first amplifier stage, a comparator stage coupled to the first amplifier stage and to an adjustable reference signal for comparing the amplitude of the output of the first amplifier stage with the reference signal and providing an error signal whose parameters depend on the relation between the amplitudes of the amplifier output and the reference signal, a multistage controllable attenuation arrangement coupled in series between said comparator stage and the input to said first amplifier stage, said arrangement including a second amplifier stage, a controllable attenuation device and a gate circuit coupLed therebetween, said arrangement providing a path through which the gain of the first amplifier stage can be adjusted when said gate circuit is opened, the gain being increased when the output of the first amplifier stage falls and vice-versa, and means for determining the presence of a signal at the input to said first amplifier stage and for maintaining said gate circuit open when a signal is present, said determining means responding to the termination of signal at said first amplifier stage input to close said gate, the arrangement of said gate circuit being such as to inhibit further changes in attenuation of said controllable attenuation device when in the closed state, said second amplifier stage having means for retaining, upon said gate circuit achieving the closed state, the control level of the attenuation setting of the attenuation device existing at gateclosing, for application to said controllable attenuation device upon reopening of said gate circuit, whereby the gain of the first amplifier is maintained at or near the level to which it was last set from said comparator, and said controllable attenuation device is thereby arranged to be both a switched device and a variable attenuator.