Audio automatic volume control circuit

Abstract

Summary form only given, as follows. Audio communication systems working through very high ambient noise levels require some kind of automatic volume control to maintain the signal above the noise and at the same time, protect the listeners' ears from painfully loud sounds. The problem is somewhat complicated by the fact that very little distortion can be tolerated. The electron tube predecessor of this amplifier used a remote cut-off pentode for automatic gain control. No such principle is available in transistor circuits. A large number of circuits were evaluated for automatic gain control characteristics. The most successful of these circuits was an absorption or shunting method in which the base-emitter resistance of a common base amplifier is the variable circuit element. A three-winding input transformer is connected between microphone and amplifier with the third winding being connected to the absorption circuit. A portion of the amplifier output signal is rectified, filtered and fed to the control transistor as its sole source of emitter current. This results in absorption of the microphone signal in proportion to amplifier output. A 30 db increase in input signal can be reduced to less than 5 db increase in output with the total harmonic distortion being less than 5%.