PURPOSE: To ensure the detection without impairing of the linearity and even with the minute amount of the input signal, by providing the amplification circuit which apmlifies the input signal.

CONSTITUTION: Transistor Q1 forms the amplification circuit and receives the DC bias by resistances R1 and R2. At the same time, Zener diode DZ and capacitor C2 are connected in parallel to load RL. With supply of input signal vi, overlap voltage VRL of load resistance RL features the waveform with its positive half cycle clipped. Furthermore, VRL is smoothed by smoothing capacitor C2 to obtain output voltage vo. And the filter property or the nonlinear property can be obtained by inserting the element featuring the fixed frequency property or nonlinear property to the positions of resistances R3 and R4 each.