PURPOSE: To restrain the fluctuation of bias voltage of amplifier transistor and to obtain a stable action by providing a by-pass means to outflow a part of current sent to controlling polar of amplifier transistor receiving the second level signal from the discriminating circuit.

CONSTITUTION: By-pass means 8 is composed of by-pass transistor TR16 and resistors 17, 18. In this constitution, when voice sub-carrier receives a broadcast of 4.5MHz, the second level signal of 8V is outputted from discriminating circuit 4, and is guided through diode 5, 6 and trap elements 2, 3 are simultaneously actuated. The sound signal of the output of picture detection IC1 is trapped by the element 3 and only the picture signal is inputted to control polar of amplifying transistor 7. At this time, as the TR16 is made to be conductive state by the second level signal, the current made by the second level signal through diodes 5, 6 is by-passed to TR16.