PURPOSE: To improve the linearity at a point approximate to the setup control voltage.

CONSTITUTION: The setup control voltage is generated by a control circuit 10 and led to a single input of a differential amplifier 20. A feedback transistor TR Q5 is added to the amplifier 20 so as to secure the coincidence between the setup control voltage and the base voltage of a TR Q2. An input signal is led to the base of a TR Q4 via a differential circuit 40. A feedback TR Q8 is added to a differential amplifier 30 so as to secure the coincidence between the base voltage of the TR Q4 and that of a TR Q3. When the base voltage of the TR Q4 is smaller than the setup control voltage, the amplifier 30 and the TR Q8 are turned off with the amplifier 20 and the TR Q5 turned on respectively. When the base voltage of the TR Q4 exceeds the setup control voltage, both the amplifier 30 and the TR Q8 are suddenly turned on and the flow of the emitter current of the TR Q5 is prevented. Then the amplifier 20 and the TR Q5 are suddenly turned off.