| 摘要 |
PURPOSE:To obtain an optical receiving equipment having good sensitivity and wide dynamic range by constituting a negative feedback loop by means of a constant-current circuit in order that the signal current of a photoelectric converting element is not increased by the input of excessive light beam. CONSTITUTION:When the input of the excessive light beam is supplied to an avalanche photodiode 1, the output amplitude from a preamplifier circuit 3 is increased to excess to saturate. Since the output from a second peak value detection circuit 11 which detects the output amplitude becomes larger, difference between a second reference voltage 12 and the output from the circuit 11 becomes larger and the output voltage value of a second error amplification circuit 13 becomes higher. Since the current capacity i1 of the constant-current circuit 14 connected to the second error amplification circuit 13 becomes larger if the output voltage value becomes higher, the current i2 flowing in the first and second load resistances 15 and 16 of the avalanche photodiode 1 is lowered and the saturation of the output from the pre-amplifier circuit 3 can be improved, thus the negative feedback loop can be obtained. When the input of light beam is small, the current capacity of the constant-current circuit 14 is equal to zero and the output from the pre-amplifier circuit 3 is controlled so that it becomes the maximum. |
