GB1489218A - Integrated circuit amplifier having controlled gain and stable quiescent output voltage level - Google Patents

Abstract

1489218 Gain control MOTOROLA Inc 21 Nov 1974 [21 Nov 1973] 50546/74 Headings H4R and H3T In a gain control circuit an amplifier 11 has, connected to its input circuit, at 22, a variable impedance comprising a two electrode device 50 which has an impedance which is dependent on the current passing therethrough, in series with a three electrode device 54 which regulates the current passing through the two terminal devices from a voltage reference source 35 in accordance with a gain controlling signal applied to terminal 58 of the three electrode device, 54. In order to stabilize the D.C. conditions of the differential amplifier 11 of the preferred embodiment a second two electrode device 52 is connected to the base of transistor 14 so that there is no change in relative D.C. voltages of the two inputs of the differential amplifier with variation of the gain control signal. In an alternative form, Fig. 4, the reference voltage is derived from Zener diode 105, and fed to the diodes 50 and 52 via emitter followers 82, 88. Constant current sources for the emitter followers and the differential amplifiers are formed by transistors 84, 90, and 102. The output from the differential amplifier is taken via a balance to unbalance converter formed by the double collector transistor 112 and emitter follower 114. The circuit may be constructed as an integrated circuit with diodes 50, 52, formed by strapped transistors.