| 摘要 |
PURPOSE:To obtain an excellent low frequency characteristic by controlling a changeover means so as to decrease the time constant of a DC level detection circuit 10 at starting and to increase the time constant of the DC level detection circuit at steady-state thereby outputting a control signal quickly. CONSTITUTION:A capacitor C1 connected between ground and a point (q) tying noninverting input terminals of an amplifier 4 and a comparetor 6 is decreased and a switching element 9 and a capacitor C2 are connected in series between the point (q) and ground. With an optical input signal to an avalanche photodiode APD1 interrupted, a potential at points (p), (q) goes to nearly a ground potential. a voltage at a noninverting input terminal (f) of the comparator 6 is higher than a voltage Vth at an inverting input terminal (g), a transistor 7 is conducted to open a contact of the switching element 9. On the other hand, the potential at the points (p), (q) is lower, the voltage at the noninverting input terminal (f) of the comparator 6 is lower than the voltage Vth at the inverting input terminal (g) to close the switching element 9. |
