WIDEBAND AGC CIRCUIT

1,185,536. Automatic volume control. INTERNATIONAL BUSINESS MACHINES CORP. 31 July, 1967 [1 Aug., 1966], No. 35058/67. Heading H4R. A gain control circuit, for use in stabilizing the output of a phase shift feedback oscillator, comprises a series resistor 27, Fig. 2, feeding an output terminal 26 with a variable shunt impedance comprising a diode 45 and the emitter collector path of a transistor 42, whose impedances are varied by varying the D.C. flowing therethrough, the diode and transistor being in series for D.C. but in parallel with respect to the A.C. signal path. The control signal to vary the current flowing through transistor 42 is derived from an A.C. signal applied to terminal 24. Transistor 32 is normally saturated with no input to terminal 24. As the signal level at 24 increases, signal peaks cause transistor 32 to cut off for a time dependent on the signal level so that pulses of current flow through diode 38 to charge capacitor 39. The pulses are integrated in capacitor 39 to provide a D.C. signal on the base of transistor 42 to control the D.C. flowing therethrough and therefore the impedance of diode 45 and transistor 42 to increase the attenuation of the circuit. The gain control circuit is connected in a phase shift oscillator as shown in Fig. 1, with the gain control signal derived from the output of the oscillator to tend to maintain the output amplitude constant irrespective of change of frequency produced by variation of resistor 10.