Compatibility circuit for telephone headset

The present invention relates to a circuit for adapting a headset comprising an ear piece and a microphone to a telephone handset. In particular, the invention relates to an electrical interface circuit to efficiently couple a headset microphone signal to a telephone signal transmitting circuit. A selectable impedance matching circuit for matching the impedance of a telephone handset adapted for either a carbon microphone input or an electronic microphone input to the impedance of a headset microphone. The selectable impedance matching circuit includes a switch for connecting the headset microphone to the handset microphone input via a full wave bridge circuit, or connecting the headset microphone to the handset microphone input via an isolation transformer and resistor load.