Audio transmission circuit comprising a transmit amplifier in the form of a line voltage stabilizer

Claim 1

An improved transmit amplifier which stabilizes the subscriber line voltage of a telephone set connected to such line, such telephone set comprising such transmit amplifier and a high-output impedance microphone pre-amplifier, such transmit amplifier being supplied with direct voltage by the subscriber line; such transmit amplifier comprising: an input stage constituted by a feedback operational amplifier having an inverting input and a non-inverting input, the inverting input thereof being connected to the output of the microphone pre-amplifier; an output stage formed by an output transistor having an emitter resistor, such output stage being arranged in parallel with the subscriber line; and a reference voltage source for producing a reference voltage for line voltage stabilization; such improvement being characterized in that such transmit amplifier further comprises: a voltage divider formed by a first resistor and a parallel arrangement of a second resistor and a capacitor, such voltage divider being in parallel with the subscriber line and the junction of said first and second resistors being connected to the non-inverting input of said operational amplifier, the emitter resistor of said output transistor forming part of said second resistor; and a feedback circuit connecting the inverting input of said operational amplifier to the emitter of said output transistor, such feedback circuit comprising said reference voltage source and a feedback resistor.