| 摘要 |
PROBLEM TO BE SOLVED: To cancel an offset, even if the negative offset occurs in the output of a preamplifier by feeding back the output of an offset detection circuit to a phototransistor through an offset suppression circuit. SOLUTION: Optical burst signals transmitted from respective subscribers are made incident on a photodiode 1, and the optical burst signals are converted into electric current. The output current inputted to a preamplifier 2 is converted into voltage and amplified. The amplified voltage is outputted to a post-stage circuit and a differential amplifier 3. An offset detection circuit 4 detects the offset quantity generated in the output of the preamplifier 2 from the differential output of the amplifier 3. An offset suppression circuit 5 feeds back an output equivalent to an offset detected in the offset detection circuit 4 to the input part of the preamplifier 2. As a result, even if the offset generated in the output of the preamplifier 2 is negative, it can be canceled automatically. |
