Electric two-to-four wire conversion circuit for use in telephone systems

摘要

An electronic replacement for a telephone transformer for connecting a two-wire circuit to a four-wire circuit comprises a first differential amplifier having a constant gain feedback loop. The feedback loop comprises an opto-electronic coupler and a second differential amplifier. A DC source connected to the first differential amplifier by a switch produces biasing current for a photo transistor in the opto- electronic coupler. A DC supply to the line on the two-wire side is provided by a source connected to the wires by transistor circuits connected as active reactances.