| 摘要 |
PURPOSE:To prevent a PNP transistor, which constitutes a differential circuit, from being saturated even while a light emitting diode is turned OFF and to decrease a delay time between a control signal and an emission waveform by a method wherein a direct current power source is added to the light emitting diode so as to constantly feed a small direct current to it. CONSTITUTION:When a control signal 17 is at an 'H' level, a light emitting diode is turned OFF, an NPN transistor 3 is turned ON, a PNP transistor 9 is put in a cut-off state, and a current is not fed to a diode 14 and a variable resistor 13 from a positive power source line 19. But, a small current is fed through a constant current source 12, so that the potential of the tap of the resistor 13 does not become negative. By this setup, the current of the current source 12 is made smaller than that of the diode 14 while it emits light and the potential of the tap of the resistor 13 is properly set, whereby a PNP transistor 1b which constitutes a differential circuit is not put in a saturation region and consequently a delay time between a control signal and an emission waveform can be lessened. |
