| 摘要 |
PURPOSE:To protect an avalanche photo diode from damage due to the application of over voltages by providing a circuit which detects the photocurrent flowing through said diode and a bias voltage reduction circuit started by the detection circuit. CONSTITUTION:When a photo input Pin is applied on the avalanche diode APD1, it passes a photocurrent IAPD and generates a voltage drop IAPDR at both ends of a resistor R. This voltage is amplified 3, fed to a comparator 4, and then compared with a reference voltage VR. If it is over the VR, the current of a switching transistor TR5 is turned on. If the photo input Pin becomes extremely small because of input break and the like, the current IAPD increases, and the voltage drop IAPDR grows. As a result, the TR5 turns on, and the TR current Itr flows. Since the flow of this current Itr to the series resistor Rs causes a great reduction of the voltage VAPD, the avalanche photo diode is protected from over voltages. |
