PURPOSE: To improve the detecting characteristics, by connecting an element improving the switching operation between a base and a collector of at least one of transistors (TR) constituting a differential amplifier performing the phase detection or constant current TRs.

CONSTITUTION: An input terminal T1 is connected to the base of the constant current TR1 and an input terminal T2 is connected to the base of the constant current TR2. The emitter of the TRs 1, 2 is connected to a constant current source in common. The collector of the TR1 is connected in common with the emitter of TRs 3, 4, and the collector of the TR2 is connected in common with the emitter of TRs 5, 6. The TR3, the TRs 4, 5 and 6 constitute respectively a differential amplifier, the base of the TRs 4, 5, is connected to an output terminal of a phase shifter 7 and the base of the TRs 3, 6 is connected to other output terminal of the phase shifter 7. The input terminal of the phase shifter 7 is connected to the terminals T1, T2. Further, Schottky barrier diodes 10, 11 are connected between the collector and the base of the TRs 1, 2, allowing to improve the detecting characteristics.