PURPOSE: To obtain the frequency continuity with a simple constitution and to improve the symmetry, by connecting two rectifier elements having the nonlinear characteristic in parallel with a series resonance circuit oppositely in polarity.

CONSTITUTION: An input terminal 1 is connected to a low output impedance amplifier 3, its output terminal is connected to one end of a resistor R1 and the other end of the resistor R1 is connected to an output terminal 2. The series resonance circuit comprising a capacitor C and a coil L and a resistor R2 are connected in series between the output terminal 2 and ground, and diodes D1, D2 are connected in parallel with the series resonance circuit so as to be arranged oppositely in polarity. If the output signal level of the amplifier 3 is very small, the impedance of the diodes D1, D2 is very high and they are negligible. If the output signal level of the amplifier 3 is large, the impedance of the diodes D1, D2 is low and they are not negligible.