PURPOSE: To compose this circuit of parts having low withstand voltage and to provide a low-noise and high-stability DC output voltage by fixedly limiting a range for the boot strap of a voltage at each driving power source with a set limit voltage while using a comparator circuit, constant voltage generator and error amplifier.

CONSTITUTION: An amplifier A3 is provided with an output stage driven by two power sources, and a transistor Q5 controls a driving voltage Vcc 1 at a fixed value. As the comparator circuit, an operational amplifier (ideal diode circuit) A4 is provided with a dead zone depending on a set limit voltage Vlmt and compares an output voltage Vo of the amplifier A3 with the set limit voltage Vlmt. Therefore, at the time of Vo=Vlmt, the operational amplifier A4 outputs the fixed voltage and at the time of Vo>Vlmt, a voltage proportional to the Vo is outputted. An operational amplifier (error amplifier) A2 calculates error between the output voltage of a Zener diode (constant voltage generator) D3 serially connected to the output system of the operational amplifier A4 and the voltage of a driving power supply system and feeds the output back to the transistor Q5.