PURPOSE: To prevent the deterioration of a waveform at the stage of amplification by detecting the intensity of an input signal and controlling the light current to be inputted to the stage of amplification.
CONSTITUTION: The optical signal is converted into light current by a light receiving element 1. A capacitor 2 sends the only signal components to a front amplifier 8 through a variable attenuator 3. Light current 7 of a signal component passing the capacitor 2 is attenuated by the variable attenuator 3 consisting of, for example, a field effect transistor. The light current detection circuit 5 detects the average value of the signal component out from the light current flowing through resistance 4. A control voltage generation circuit 6 compares the voltage detected in the light current detection circuit 5 with the reference voltage, and generates the voltage controlling the attenuation amount of the light current 7 in the variable attenuator 3. The light current 1 which is attenuated in the variable attenuator 3 is amplified by the front amplifier 8. Accordingly, the light current 7 to be inputted to the front amplifier 8 can be reduced, and the gain saturation in the front amplifier 8 can be prevented.
YAMAMOTO HIROAKI
UCHIUMI KUNIAKI
FUJITO KATSUYUKI
