Automatic Gain Control(AGC) is a method used in audio technology, hearing aids, RF technology, mobile radio and also in optical transmission technology, among others, and variable gain amplifiers( VGA) are used to control their gain.
The AGC method is a control circuit and uses a control voltage obtained from the useful signal. This control voltage is used to control the gain of one or more amplifier stages. For this purpose, the control signal obtained adapts the signal level to a specified signal level by means of feedback.
This signal level adjustment can be expressed in a signal level increase, i.e. a gain, but also in a signal level reduction, depending entirely on the signal level of the useful signal. The AGC technique is used, among others, in RF amplifiers and IF amplifiers as feed-back control and in AF amplifiers as feed-forward control.
Characteristics of AGC
Automatic gain control can have different characteristics. Thus, its course can be linear, progressive or degressive. In the first case, the gain increases linearly with the AGC level; in the second case, it changes nonlinearly according to a given function, for example, human auditory perception.
In optical networks, automatic gain control occurs, for example, in optical transmission links to compensate for any differences in light levels so that receiving devices always have the same input level and are not overdriven.
In hearing aids there are separate AGC procedures for the output level, the AGCo for output and the AGCi for input. The AGCo determines the control threshold for the output sound pressure and limits the maximum output sound pressure. The AGCi value is the control threshold for the input sound pressure, which it limits to a maximum value. Both AGC values are described in the DINIEC 60118-2 standard.