Quasi-resistive battery feed for telephone circuits

Abstract

An electronic circuit simulates the direct current characteristics of the hybrid transformer portion of the telephone system call-handling equipment. The circuit is connected across a two-wire telephone subscriber loop and supplies a loop current that is proportional to the difference between a constant current and a reference current. The circuit contains a voltage sensing circuit which senses the voltage across the loop and transfers the sensed voltage across a reference resistor, thereby developing a reference current through the resistor which is proportional to the voltage across the subscriber loop. An integral current generator supplies a constant current to the circuit. An integral current subtractor, connected between the output of the voltage sensing circuit and the current generator subtracts the constant current from the reference current. The resulting current forms the input current to an integral current amplifier. The current amplifier amplifies the current from the current subtractor, thereby producing an amplified current which is proportional to the reciprocal of a constant plus the resistance of the subscriber loop. The current supplied the loop by the circuit is the output current from the current amplifier. The electronic circuit forms part of a larger integrated circuit which is a complete replacement for the hybrid transformer.