US Patent RE40897 Constant current line circuit with class features

A line circuit utilizing a differential amplifier whose output is regulated to zero volts D.C. (when the subscriber's set is off-hook) by a negative feedback circuit, for controlling a voltage controlled power supply which provides a constant current feed to the subscriber's loop. The differential amplifier also transmits audio signals received from the subscriber's loop directly to a PABX via an unbalanced terminal. When the loop range exceeds a predetermined threshold, the circuit reverts to a voltage feed operation thereby allowing operation over long loop lengths and the ability to provide on-hook CLASS services.