Voice circuit of telephone

Claim 1

A voice circuit of a telephone comprising: a transmitter; a pair of telephone lines through which a DC bias current flows; a regulating power supply circuit, connected between said pair of telephone lines, for generating a predetermined voltage on the basis of the DC bias current flowing between said telephone lines; a first amplifier which is made operative by the output voltage of said regulating power supply circuit and amplifies an output of said transmitter; a differential amplifier having an inverting input terminal to which an output of said first amplifier is supplied, an noninverting input terminal, and an output terminal; a line driver, connected between said pair of telephone lines, for transmitting a first signal onto said telephone lines in response to an output signal of the differential amplifier, said first signal being divided into a plurality of components; an impedance setting circuit, connected between said pair of telephone lines, for supplying an output to the noninverting input terminal of said differential amplifier and for feeding back part of said signal components through a positive feedback loop, said feedback loop including one of said telephone lines, said impedance setting circuit, said differential amplifier and said line driver, and wherein said impedance setting circuit adjusts the level of the signal components transmitted to said telephone lines; said impedance setting circuit consists of first and second resistors which are connected in series between said pair of telephone lines and a capacitor which is connected between a node of said first and second resistors and the noninverting input terminal of said differential amplifier; a second amplifier which is made operative by the output voltage of the regulating power supply circuit and amplifies a second signal, other than said first signal, which is received from the telephone lines; and a receiver to which an output of said second amplifier is supplied.