PURPOSE: To quicken a clamp operation by applying negative feedback to a differential amplifier by means of a 2nd switch means at the same time the differential amplifier is disconnected from a signal path by a 1st switch means.

CONSTITUTION: First and second switches 4, 5 are turned off to disconnect a differential amplifier 3 from a signal line and a 3rd switch 6 is turned on to short-circuit an anti-phase input with an output of the amplifier 3. Thus, feedback is applied to the amplifier 3 always at a GND level, and saturation of the output by a positive drive power supply voltage and interrupting state by a negative drive power supply voltage are avoided. Furthermore, Since feedback is applied by using a voltage for clamping, the clamp voltage is immediately sent to a signal line by turning on the switches 4, 5 and turning off the switch 3 for 2nd clamping thereby increasing the clamp operation speed.