GB1485092A - Amplifier with gain control - Google Patents

Abstract

1485092 Transistor amplifiers; substation circuits PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 28 Oct 1975 [31 Oct 1974] 44360/75 Headings H3T and H4K An amplifier with gain controlled by a current, having signal input terminals 4, gain control terminal 1 and common terminal 3, comprises a transistor T2 with signal output 2 coupled to the collector and supply and with its base D.C. coupled to gain control terminal 1, e.g. via the base-collector junction of transistor T1, with a shunt to common line 3 via a semi-conductor junction such as the base-emitter junction of transistor T1, or via a diode or diode connected transistor, in series with resistance R1 having the signal input across it, in such a way that the D.C. component Iu of current through the collector-emitter path of transistor T2 is proportional or equal to the control current I0 and large compared with the base current of T2. In the simplest embodiment (Fig. 1, not shown), only transistors T2 and T1 are present, T1 being diode connected, the circuit acting as a current mirror, keeping the currents through the two transistors proportional or preferably equal, largely independent of temperature, by means of identical transistors and proportional loads R1, R2. The gain of transistor T2 is determined by its D.C. current Iu and therefore by control current Io; the input impedance in the absence of R1, is high, allowing R1 to be dominant, while the input impedance to control current is low. Mismatch due to the base current of T2 may be compensated, improving current balance and temperature compensation, by coupling the base of T2 to the collector of T1, no longer diode connected, and driving the emitter of T2 from the base of T1, feeding R2 via a third transistor T3 diode connected (Fig. 2, not shown). In a telephone circuit, see Fig. 3, in which the control current Io is made to vary inversely with the energizing current from the telephone line by external means, the gain of transistor T2 is again controlled, with the base currents of both T1 and T2 supplied by auxiliary transistor T4: the input impedance at terminal 4 in the absence of R1 is then higher by a current gain factor # than in the earlier embodiments, allowing more latitude in R1. The output is further amplified by Darlington pair T5, T6 and, if the input 5 is a microphone, is applied to the telephone lines 6, 7 also acting as supply. I.C. input 5 is from an incoming line, the output is taken to a receiver.