GB613141A - Variable gain amplifier - Google Patents

Abstract

613,141. Automatic gain control. BENDIX AVIATION CORPORATION. March 5, 1946, No. 6894. Convention date, March 12, 1945. [Class 40 (v)] An amplifier is provided with two gain control means which increase and decrease its gain respectively with increases in input signal level, and the second means are arranged to modify the operation of the first to reduce the effects of high background noise level occurring between periods when the input is due to desired signal energy. In the amplifier described, the pentode amplifier 20 feeds the push-pull output stage 46, 48. The output from stage 20 is also fed to a triode 90 feeding a full-wave rectifier 106 producing a positive bias for the grids of valves 46, 48 across its load resistance 78, which is provided with an adjustable tapping 79 to vary the amount of volume expansion. The output transformer 54 of the push-pull stage has a winding 55 feeding a second fullwave rectifier 85 provided with an adjustable delay bias from resistance 66 connected in series with resistances 64, 68, 70 across the H.T. supply. When the output signal is of sufficient amplitude to overcome this delay bias, a negative bias for the output valves is produced across the load resistance 76 resulting in volume compression of the output signals. At the same time the volume expansion circuit is disabled by the application of the same negative bias to the valve 90.