Further, the circuit structure of (b) of FIG. 11 is an example in which the control signal generating section 1 is formed, by providing a constant voltage source 1E instead of the above constant current source 1A and variable resistor 1B, and a variable resistor 1F instead of the resistor 1D. In this control signal generating section 1, a current output as the Tr/Tf control signal Sc can be contro