| 摘要 |
PURPOSE:To obtain an excellent S/N ratio and to improve receiving specific sensitivity by always detecting the output amplitude of a preamplifier, negatively feedback controlling a cathode voltage to be applied to an APD so as not to become a predetermined value or higher, detecting the output amplitude of a gain variable amplifier, and negatively feedback controlling a gain so as to become a constant value. CONSTITUTION:If an optical input level is low and the output amplitude of a preamplifier 2 is less the allowable maximum input amplitude of a gain variable amplifier 3. The amplification factor of an APD 1 is fixed to a value in which its S/N ratio becomes maximum. If the level is high, the cathode voltage of the APD 1 is negatively feedback controlled so that the output amplitude of the preamplifier 2 is fixed to the allowable maximum input amplitude of the amplifier 3. On the other hand, a second negative feedback loop is formed of the amplifier 3 and a gain controller 5, and the gain of the amplifier 3 is controlled so that the output amplitude of the amplifier 3 becomes constant. That is, if the level is low, only the gain of the amplifier 3 is controlled, while if the level is high, only the amplification factor of the APD 1 is controlled. |
