voltage vb applied to the second input terminal 16a 2 sets the oscillation frequency f0 of vco 16 to a prescribed center oscillation frequency fc when the voltage va on the first input terminal 16a 1 differs from the reference voltage vb the oscillation frequency fo varies from the center oscillation frequency fc the operation of the switch circuit 126s will be explained hereafter in reference to fig4 when masking signal sm is not supplied to the control terminal 126k a of the constant current source 126k in the switch circuit 126s the constant current source 126k supplies a prescribed current to the differential amplifier circuit comprised of the resistors 126a and 126b and the transistors 126c 126d 126e 126f 126g 126h 126i and 126j thus the switch circuit 126s of the differential amplifier type activates in the activation state the differential amplifier circuit operates to equalize the base potential of transistors 126i to the base potential of transistors 126g as a result the reference voltage vb of the dc voltage source 124 is supplied to the first input terminal 16a 1 of vco 16 through the switch circuit 126s the switch circuit 126s of the differential amplifier type has a remarkable efficacy to minimize the potential difference between an input end such as the base of the transistor 126g and an output end such as the base.