Balanced DC supply for telephone subscriber circuit - has complementary transistors in series with wires and controlled by differential amplifier with battery and inter wire voltage inputs

The DC supply circuit, for a telephone subscriber circuit, is located between the telephone line and the supply battery. It produces a constant current with the aid of two complementary transistors each in series with one of the wires in the 2-wire telephone line. The base current of these two transistors is modified by a correction circuit so that they operate close to their working point and are balanced. The correction circuit consists of a differential amplifier whose two inputs receive (via two resistive voltage dividers) the voltage across the battery's terminals and the voltage between the wires.