PURPOSE: To perform full-wave rectification in the center of an input waveform at all times by supplying the output of an extracting circuit which extracts only the DC voltage of an input voltage as the reference voltage of an amplifying circuit and a switch means.

CONSTITUTION: The extracting circuit 9 which extracts only the DC component of the input voltage is provided and the output of this extracting circuit 9 is supplied to the '+' terminals of amplifying circuits 1 and 8 and switches Φ3 and Φ6. The waveform of the input voltage passes through the extracting circuit 9 to become a DC power source corresponding to the center value of the input waveform. For example, when there is an offset, a DC voltage consisting of only the offset component is obtained and this voltage is supplied as the reference voltage of the amplifying circuits 1 and 8 and to the switches Φ3 and Φ6. Consequently, the full-wave rectification is performed in the center of the waveform of the input voltage at all times.