Electronic hybrid having synthesized impedance circuitry

Claim 1

A circuit for coupling a two-wire transmission path to a four-wire transmission path, said circuit comprising: a first amplifier having inputs coupled to said two-wire transmission path and an output coupled to one side of said four-wire transmission path; first and second circuitry means for each providing combined battery and voice signal injection to said two-wire path whereby a.c. voice signals are combined with d.c. battery potential, each having an input terminal coupled to the other side of said four-wire transmission path and each having an output terminal; a first battery feed impedance coupled between said first circuit output terminal and one side of said two-wire path; a second battery feed impedance coupled between said second circuit output terminal and the other side of said two-wire path; each of said first and second circuits comprising a differential input amplifier having first and second inputs and an output, a first resistor coupled between said input terminal and said first input, a first feedback impedance coupled between said output terminal and said first input: circuit means coupling said output to said output terminal, a second resistor having one end coupled to a voltage source terminal and its other end coupled to said output terminal via said circuit means, a second feedback impedance coupled between said second resistor other end and said second input, whereby said first and second circuit means each operate to provide an output impedance at the respective said output terminal which is approximately zero for d.c. and is a predetermined non-zero value at voice frequencies.