Active impedance line feed circuit

摘要

Normal operating range of an active impedance line feed circuit is extended to at least that of a prior art passive resistance line feed circuit. The active impedance line feed circuit includes tip and ring feed resistors in series with outputs of tip and ring amplifiers for coupling a.c. signals to a two wire communication line with an effective source impedance as defined by an a.c. feedback network in response to differential currents in the communication line. Effective output direct current incremental resistances of the tip and ring amplifiers are controlled by a d.c. control circuit connected across the tip amplifier. In an instant of an OFF HOOK communication line of less than a certain resistance a predetermined constant direct current is supplied by controlling the output direct current incremental resistances to be high. In an instant of the communication line resistance being too high, as in the case of a longer line, the output direct current incremental resistances of the tip and ring amplifiers are controlled to approach a few ohms such that at least a minimum energizing direct current is suppliable from the active impedance line feed circuit over an extended range of communication line.