PURPOSE: To improve the linearity of the frequency discrimination characteristic for input of a large signal by adjusting the emitter resistance value of a differential pair of transistors TR constituting a frequency discriminator circuit to extend the input dynamic range.

CONSTITUTION: A DC voltage is received by one of three input terminals, and signals which have an arbitrary frequency and a certain phase difference are received by two other input terminals, and these signals are receives by a differential pair of TRs 21 and 22 and a differential pair of TRs 23 and 24 respectively, and output currents of both differential pairs are added, and the output is subjected to full-wave rectification by a full-wave rectifying circuit and is outputted through a low pass filter 17.