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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.
Frank J. Clement and Bell Labs received a patent in 1969 for a multiple-station conference telephone system that switched its output to the loudest input. [7] The next year, Emil Torick and Richard G. Allen were granted a patent for an "Automatic Gain Control System with Noise Variable Threshold", an adaptive threshold circuit invention with its patent assignation going to Columbia ...
Automatic gain control circuits are intentionally designed to actively change the overall gain in response to the level of the input, resulting in a transfer function that may vary over time. Gain compression on the other hand is a consequence of analog amplifier circuit non-linearities that are generally undesired.
The loop gain is a product of the very high amplifier gain and the very low bridge ratio. [13] In Hewlett's circuit, the amplifier is implemented by two vacuum tubes. The amplifier's inverting input is the cathode of tube V 1 and the non-inverting input is the control grid of tube V 2.
Look-ahead is implemented by splitting the input signal and delaying one side (the audio signal) by the look-ahead time. The non-delayed side (the gain control signal) is used to drive the compression of the delayed signal, which then appears at the output. This way a smooth-sounding slower attack rate can be used to catch transients.
Automatic gain control, intentional non-linearity and incidental non-linearity limit the output amplitude in various implementations of the oscillator. The circuit shown to the right depicts a once-common implementation of the oscillator, with automatic gain control using an incandescent lamp.
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It functions strictly on the signal strength, such as when a television mutes the audio or blanks the video on "empty" channels, or when a walkie-talkie mutes the audio when no signal is present. Carrier squelch uses receiver Automatic gain control (AGC) to determine the squelch threshold. Single-sideband modulation (SSB) typically uses carrier ...