PURPOSE: To control to change the duty ratio of an output waveform to an ambient temperature change by matching to the temperature change of-the pinchoff of an FET and changing a bias voltage by a temperature compensating circuit.

CONSTITUTION: A voltage converting part 2 is connected to the source side electrode S side of an FET 1, a voltage converting part 2 is connected, and on the other hand, a load circuit 4 such as a laser diode is connected to a drain electrode D side. The converting part 2 includes a temperature detecting part 3, a stabilized source voltage VS, in which the value is changed in accordance with the temperature detected by the detector 3, is supplied to a source electrode S of an FFT1. The voltage VS become lower at the time of the low temperature, the width of a voltage Vgs between gate sources is made narrow, on the other hand, the voltage becomes higher at the time of the high temperature, the width of the voltage Vgs is made wider and as such, the change is executed for the temperature. The temperature variation of the voltage VS is adjusted so as to be coincident to the variation of a pinchoff voltage Vp of the FET1. Thus, for the temperature change, the voltage VS is changed for the temperature change following the voltage Vp, and thus, the slice level of the input waveform is made always constant to the temperature.