Telephone line circuit for 4 wire - 2 wire bidirectional conversion - uses two differential amplifiers and DC supply without requiring perfect filtering

摘要

The telephone line circuit has a two wire symmetrical line (64,65) for connection to a phone and an asymmetrical output line wire. A first differential amplifier has its inputs connected through equal value resistances to the symmetrical line wires, the resistance values being equal to at least ten times the phone impedance when unhooked. The output of the amplifier is connected to the output line wire to apply output signals to the line. The output at a second amplifier is connected to one wire of the two wire lines through a first resistance whose value is chosen to apply to one wire of the two-wire line a signal to compensate in the first amplifier the signal applied to the input connected to the other wire from the second amplifier output. The latter's input is connected to the input wire line to receive input signals. The circuit is supplied through a d.c. source of e.q. 24 volts.