| 摘要 |
PURPOSE:To keep a drive level of a laser diode constant in high speed and to obtain a laser diode output with a low distortion by providing a DC level fixing circuit fixing a DC level of a modulation signal input and a DC coupling circuit superimposing a variable DC level on an output of the said DC level fixing circuit. CONSTITUTION:A base band video signal inputted to a modulation input terminal 5 is subject to DC limit for its pedestal level by a clamp circuit 20, the result is given to an inverting input of a differential amplifier 21, from which the signal is outputted to a resistor 6 as a negative modulation signal and fed to the laser diode 1. On the other hand, a DC bias current driving the laser diode 1 controls a non-inverting input of the differential amplifier 21 by setting a voltage at a bias set terminal 22, a DC bias voltage superimposed on the modulation signal is impressed to a resistor 6, transistors 3, 4 of Darlington connection are drives, causing a DC lighting level of the laser diode 1 to be set. Thus, the base band video signal is operated in the range of a constant drive current versus light emitting output characteristic of the laser diode 1. |
