A channel unit interface circuit for interconnecting a two-wire, bidirectional signal transmitting path and a four-wire signal transmitting path in a communication system includes a tip terminal transconductance amplifier driver, a ring terminal transconductance amplifier driver, a voice sense amplifier, a balance network, A/D signal processing circuit, D/A signal processing circuit, filter network, a voice codec/PCM filter, a channel logic circuit, and a DC-to-DC converter. The tip terminal transconductance driver is formed of a first switching operational amplifier, and the ring terminal transconductance driver is formed of a second switching operational amplifier. Each of the first and second switching operational amplifiers is formed of a low voltage transconductance amplifier, high voltage comparator, a filter network, and a feed forward compensation amplifier.