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.