PURPOSE: To build a circuit capable of an adequate supply of currents under a heavy load with noise voltages suppressed low by a method wherein time constant circuits are provided in a buffer front stage and the time length for a buffer to be turned on is subject to the time constant when a switch control signal is changed over in its polarity for the turning-on of the buffer.

CONSTITUTION: Time constant circuits 61 and 62 are connected between an input terminal 1 and buffer 7, and the buffer 7 is turned on after the passage of time from the inversion of a switch control signal (a), with the length of the time corresponding to the time constant provided by the time constant circuits 61 and 62. This design enables a MOSFET with a great driving capability to be used as the buffer 7, which means a lowered resistance to the output of the buffer 7. This also means that there will be no increase in the output terminal voltage when the buffer 7 is turned on under a heavy load, which ensures an adequate supply of currents. Noise voltages will be kept low because of the time constant governing the time the buffer 7 requires before it is turned on when the polarity is changed over of the switch control signal (a).