To enlarge a noise margin by providing an additional current source circuit for normally flowing a current to a latch part in addition to a constant current source.

An additional current source circuit S1 connected to an emitter side of a transistor Q4 on a latch circuit side consists of, for example, a current mirror circuit and constantly flows a current Iadd of several to 10 percent of its operation current Io to transistors Q3 and Q4. Since the current constantly flows to the transistors Q3, Q4 or the like which constitute the latch part from the additional current source circuit S1, the transistors Q3 and Q4 of the latch part are not completely turned off even switched and a distortion of an output waveform is not caused. In this case, actually the transistors Q3 and Q4 are virtually not turned off because the transistors enter a latch operation state earlier than they are turned on from the state in which they are turned off in accordance with an amount of the additional current, and the distortion of the output waveform is reduced.