PURPOSE: To detect a peak position at an input voltage being a negative voltage below zero by deciding the polarity of the 1st and the 2nd diodes being at the same polarity between an output and a non-inverting input of an operational amplifier and giving an input voltage between the non-inverting input and a common.

CONSTITUTION: The output of the operational amplifier OP1 is connected to an anode of a diode D1. The input voltage (ei) is applied between the non-inverting input and a common of the operational amplifier OP1, and an output voltage l0 is extracted between the output of the operational amplifier OP1 and the common. As the input voltage (ei) increases, a voltage between an inverting input and the non-inverting input of the operational amplifier OP1 is decreased gradually less than the forward voltage of the diode D2 and reaches the voltage finally, and when the voltage exceeds the value, the output voltage (eo) of the operational amplifier OP1 rises positively, the diode D1 is conductive, the diode D2 is nonconductive, the circuit restores to the state similar to the first state. Thus, the peak position is detected.