To generates a saw tooth wave synchronous with a synchronizing signal by inputting the same synchronizing signal as a signal outputted from a differential amplifier to a buffer.

A synchronizing circuit 5 inputs a synchronizing signal having the same value as a signal outputted from the differential amplifier 1 from an external device to the buffer 2. The circuit 5 is constituted of a pnp transistor (TR) Q6, npn TR Q7 and a current source 15. The emitter of the TR Q7 and the current source 15 are conneced in series between a power supply Vcc and a ground power supply and a synchronizing signal is inputted to the base of the TR Q7. The TR Q6 connects its base to a node between the TR Q7 and the current source 15, connects its emitter to the base of an npn TR Q3 and connects its collector to the ground. If an external synchronizing signal falls when a capacitor C1 is charged with electricity from the current source 14, an npn TR Q2 is turned off and discharge from the capacitor C1 is started. Thereby the base voltage of an npn TR Q1 falls, the TRs Q1, Q5 are turned off and the TR Q2 is turned on, starting charging.