PURPOSE:To improve the detection efficiency by giving a constant voltage as a non-inverting voltage of a differential input signal to form a reference voltage and comparing it with an input voltage having a voltage of any higher value. CONSTITUTION:Differential input signals VI1, VI2 are fed respectively to base input terminals of transistors (TR) Q14, Q15. The TRs Q14, Q15 constitute respectively a level shift circuit and an output of each level shift circuit is fed to the bases of TRs Q11, Q12. Further, a combined voltage of resistors R16, R17 connected between emitters of the TRs Q15, Q16 is fed to the base of the TR Q13 and a voltage being the addition of a constant voltage to an in-phase voltage of the differential input signal is fed to the TRs Q11, Q12 as the reference voltage. Thus, when the amplitude of base potentials VB1, VB2 of the TRs Q11, Q12 corresponding to the input signal exceeds the base potential VB3, a detection output is obtained. Since a detection output waveform is obtained twice per one input period, a high detection sensitivity is obtained.
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