| 摘要 |
PURPOSE:To prevent light emission of an LED or of a laser diode and simplify detection of input signal cutoff on the side of a photoreceiver by making voltage between a gate and a source of an FET for driving less than pinchoff voltage at the time of input signal cutoff for making a current not to flow between a drain and a source. CONSTITUTION:In a circuit driving a light emitting element 1 by a field-effect transistor 2, in which Zener diode 3 is connected with a source, an input signal is branched to be added to the field effect transistor 2, cutoff of the input signal is detected to be added to an input cutoff detection circuit 4, which outputs a detection signal, and output of a transistor 5, which turns on at the time of the input signal cutoff, is added to the source of the field effect transistor 2 being controlled by the detection signal of output of the input cutoff detection circuit 4. That is to say the transistor 5 is operated only at the time of input signal cutoff for continuing to make a current flow to the Zener diode 3 to keep source potential of the field effect transistor 2 same with the time of normal operation. As a result, the potential between a gate and the source of the field-effect transistor 2 becomes less than pinchoff voltage even at the cutoff time of the input signal so as to prevent luminescence of a light emitting element. |
