Telephone circuits

Details Telephone circuits Telephone circuits

H - Electricity

04

M

379/82

H04M 1/60 (2006.01) H04M 1/58 (2006.01)

Patent

CA 1257724

ABSTRACT TELEPHONE CIRCUITS A telephone circuit for use in a telephone handset which may be implemented substantially in low voltage technology with a minimum of high voltage components comprises a signal node (1) for receiving a signal voltage; a microphone signal input (20); an earphone amplifier (21-25) having an input (101) and an output (26); a first impedance (11) coupled between the signal node and the input of the earphone amplifier; means (18) for combining a microphone signal fed to the microphone signal input with a voltage derived from the signal node via a second impedance (10) to provide a combined output signal at its output, the output of said combining means being coupled to the input of said earphone amplifier; and means (14) for comparing the combined output signal with a voltage representative of current flowing at the signal node, and output voltage provided by the comparing means being used to control the current flowing at the signal node, and the combined output signal being in antiphase with the signal voltage at the signal node so as to tend to cancel the microphone signal at the input of the earphone amplifier.

519847

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