| 摘要 |
PURPOSE:To always obtain a nearly flat frequency characteristic by operating an AGC circuit and also controlling an applied voltage to a varactor element to a proper value even when a front end circuit comprising an avalanche photodiode (APD) added with the AGC circuit and a high input impedance circuit. CONSTITUTION:An incident signal light 1 of a specific wavelength modulated by a pulse signal of a specific value is converted into an electric signal by an APD2 and becomes an output signal 22 after passing through a coupling capacitor 13, a high input impedance circuit 20 and an equalizing circuit 21. A part of an output signal 22 is extracted as a monitor use and peak detection is applied by a detection circuit 29. Then a peak detection output and an output of a reference voltage generator 23 are compared by a differential amplifier 24 to obtain an error signal 25. An APD bias circuit 26 is controlled by the error signal 25 to change the APD bias voltage 27 thereby changing the multiplier rate of the APD2, resulting that the control is applied to keep the output signal 22 always to a constant level. Since the characteristic of the equalizing circuit 21 is changed in response to the change in the band characteristic of the front end circuit in this case, the error signal 25 controls simultaneously a varactor element bias circuit 28 to adjust the band characteristic of the equalizing circuit 21. |
