GB1290520A - - Google Patents

Abstract

1290520 Automatic gain control S KUDELSKI 23 Dec 1969 [26 Dec 1968] 62710/69 Heading H4R In an automatic gain control circuit the rate at which the gain increases in response to a decrease in output signal level is increased when the output signal level has been below a predetermined level:for more than a predetermined time. As described a variable impedance network 10 controls the signal applied to an amplifier 11 in dependence on a control signal derived from the amplifier output by a full wave rectifier circuit including transistors T1 and T2, fed with antiphase amplifier output signals which charge a capacitor C1, the voltage across which controls the impedance of the network 10. The capacitor C1 has a first discharge path R1 and second discharge path including R3 and transistor T4 which is normally non- conductive, rendering the discharge path inoperative, but is rendered conductive, to lower the discharge time of the capacitor C1, when the signal level has been below a predetermined level for more than a predetermined time. Transistor T4 is normally held non-conductive by the conduction of transistor T3 in response to the signal from amplifier 11 applied to its base. When the amplifier output level falls transistor T3 ceases to conduct allowing capacitor C2 to charge and, after a predetermined time, turns on transistor T4. The variable impedance network as shown in Fig. 3, includes a diode bridge 21 as a variable shunt impedance to earth from the signal path between terminals 23 and 24 with the D.C. control current for the bridge derived from a circuit including a logarithmic network 29 to provide a variation of gain following a logarithmic law relative the control voltage at terminal 25.