Constant DC supply for telephone instrument - has feed into one line with balancing impedance connected to other line

摘要

The constant current source (1) feeds into one line (La) of a two wire connection to a subscriber instrument (Tln). The return to the dc source negative from the other line (Lb) is via an impedance, typically a choke, to provide symmetry between the lines and supply potential. The impedance of the source (1) and choke should be equal. In a practical circuit, the choke is simulated by a capacitor and transistor configuration. A transistor (Ts1) is connected as an emitter follower and may drive a second transistor (Ts2). The emitter potential tends to be held constant by the capacitor (C1) thus any change in current through the transistors due to a change in applied potential is slow and simulates the response of an inductance.