| 摘要 |
PURPOSE:To improve the maximum light receiving sensitivity by constituting a negative feedback circuit so that a bias voltage fed to a photodetector is always at a prescribed level or above. CONSTITUTION:The bias voltage of an avalanche photodiode (APD) 1 is divided by resistors 11, 12 and a divided bias voltage is given to a comparison amplifier 13 via a resistor 14 and compared with a reference voltage (e) and the result is amplified. The amplified signal is given to a DC/DC conversion circuit 3 via transistor (TR) 15 in wired OR connection to the emitter of a TR 5 for a gain control voltage input. In increasing the optical input power and an APD current, since the output impedance of the DC/DC conversion circuit 3 is not sufficiently low, the APD bias voltage tends to go down, but the input point (c) of the circuit 3 is increased for its level to increase the voltage at an output point (d) by a negative feedback, then the effect of the output impedance of the DC/DC conversion circuit 3 is reduced to keep the APD bias voltage constant. |
