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AutomaTIc Gain Control （AGC） Overview
A microphone is typically used in an environment where the level of the audio source is unknown. The
LM4935 features an AutomaTIc Gain Control （AGC） circuit that addresses this situaTIon by compensaTIon
for significant variations in level from a microphone’s input. It maintains a fixed level during recording or
playback. The following scenarios showcase the real world benefits of the AGC circuit：
Scenario 1：
A user talks too loudly or too close to the microphone. This causes a clipped voice signal at the output of
the microphone preamplifier.
Without AGC
The user at the receiving end hears a clipped signal. This clipped signal sounds distorted. Further， it may
damage the receiving-end transducer or may cause hearing loss.
With AGC
The AGC automatically lowers the microphone preamplifier gain. This prevents voice signal clipping. The
resulting undistorted， unclipped voice signal is then presented to the user on the receiving end.
Scenario 2：
A user talks too softly or too far away from the microphone.
Without AGC
The resultant low-level voice signal may not be heard clearly or at all by the user at the receiving end.
With AGC
The AGC will automatically increase the gain of the microphone preamplifier to a level that is heard clearly
by the user at the receiving end.
As shown in the scenarios above， the AGC operates by automatically adjusting the gain of the microphone
preamplifier to maintain a pre-defined target level at the amplifier‘s output. The AGC loop is shown in
Figure 1.
