GB1420936A - Gain control arrangements - Google Patents

Abstract

1420936 Gain control; transistor amplifying circuits PYE Ltd 14 Feb 1973 [29 March 1972] 14809/72 Headings H3T H4R and H4J In a gain control circuit, particularly for video signals, the input signal is fed through a phase splitter circuit 5 to provide two signals 10, 11 in antiphase which are fed to similar gain controlled differential amplifiers 6, 7 the outputs from which are fed respectively to the inverting and non-inverting inputs of a differential amplifier 8. In the resulting output, 2, distortion due to non-linearity of the gain controlling amplifiers 6, 7, is cancelled. In the preferred arrangement the gain controlled amplifiers 6, 7 comprise emitter coupled pairs (T2, T3), Fig. 2 (not shown), with the signal fed into the common emitter circuit and taken out of one collector circuit, in which the division of current between the two transistors is controlled by a control voltage applied to the bases of the transistors. The phase splitter comprises an emitter coupled pair of transistors T5, T6, Fig. 3 (not shown), with low impedance output circuits (T7, T8) and negative feedback to stabilize the gain. The control voltage to vary the gain of the circuit, by varying the current division in the emitter coupled pairs of amplifiers 6, 7, is preferably fed via a non-linear circuit, Fig. 6a, in which the non-linear current voltage relation of diodes D1 and D2 compensates for the non-linear gain control characteristic of the amplifiers 6 and 7. Alternative non-linear circuits are described, Figs. 6b, 6c, 6d, 6e (not shown). Two similar circuits may be combined using a common differential amplifier (108), Fig. 8 (not shown), and with the amplifiers connected to provide complementary gain control characteristics to give a " fade over " characteristic from one input circuit to the other.