Importance

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Abstract

A telephone headset interface circuit is coupled to a telephone line to extract power for use by a headset amplifier and is particularly adapted for use with telephone handsets having dynamic or electret microphones. The telephone line connects a telephone to a service system, such as a Central Office, a PBX system, or a Key system. The interface circuit includes a power extraction circuit having a plurality of circuit portions, each one coupled to a corresponding one of the telephone line wires for providing a predetermined output voltage, so that substantially identical electrical impedance characteristics are introduced to each such wire. A power compensation circuit is coupled to both a first and second pair of wires of the system for controlling the power extraction circuit to provide the predetermined output voltage in response to the presence of AC voltage on the second pair of wires, such that the power compensation circuit controls the power extraction circuit to maintain substantially the same electrical impedance characteristics to the first pair of wires regardless of the presence or absence of AC voltage on the second pair of wires. An isolation circuit is coupled to the outputs of the power extraction circuit portions for preventing shunt loading of the first and second pairs of wires and for providing electrical impedance characteristics to the first and second pairs of wires representative of an open circuit.

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First Claim