PURPOSE: To decrease the influence of power source by making the voltage between base emitters of a transistor constant with the assembling of a reverse polarity transistor and the impression of a direct current off-set voltage and adding the circuit composed of a transistor, capacitor and adding the circuit composed of a transistor, capacitor and resistance to the feedback base of a current supplying circuit.

CONSTITUTION: A current supplying circuit 26 composed of resistances 1∼6 and transistors 16∼18, a current supplying circuit 27 composed of resistances 8∼13 and transistors 19∼21 and the section between both circuits are connected by a resistance 7 and capacitors 14 and 15. Terminals 22 and 23 of current supplying circuits 26 and 27 are connected to a telephone line, and terminals 24 and 25 are connected to a power source 28. By such a constitution, a direct current impedance is not changed by a loop electric current and comes to the current supply circuit of a constant resistance. The capacitor 15 is provided so that an alternating current signal cannot be fed back to the base of transistors 18 and 21 and the differential alternating current impedance of the current supply circuit can be heightened. Transistors 16 and 19, resistances 3 and 10 and the capacitor 14 are the circuits to attenuate the power source noise.