PURPOSE: To simplify the constitution and to form a signal identifier identifying surely a signal by controlling a bias voltage reversely in response to the signal level.

CONSTITUTION: A comparator 6 is designed to have a characteristic that the higher the input voltage the more the bias current IB flows. When a large amplitude signal is inputted in such a case, that is, when the level of a data is at '1', the large bias current IB flows to the comparator 6 and a voltage Vi at a connecting point is lowered by a voltage being a product between the current IB and a resistor 2.When a small amplitude signal is inputted, the bias current IB is decreased, then the voltage Vi at the connecting point rises. Thus, a threshold voltage Vrefis set always at the center of the waveform independently of a large or small amplitude input.