Differential amplifier circuits that receive input signals fed through external terminals are served with a first operation voltage and a second operation voltage through a first switching MOSFET and a second switching MOSFET, said first and second switching MOSFETs are turned on by a bias voltage-generating circuit when said input signal is near a central voltage of said first and second operation voltages, control voltages are formed to turn either said first switching MOSFET or said second switching MOSFET on and to turn the other one off to produce a corresponding output signal when the input signal continuously assumes said first voltage or said second voltage for a predetermined period of time, thereby to supply an input signal of a first amplitude corresponding to said first operation voltage and said second operation voltage as well as an input signal of a second amplitude corresponding to a predetermined intermediate voltage between said first operation voltage and said second operation voltage.