PURPOSE: To simplify the circuit configuration of the title circuit and to eliminate the necessity of offset voltage adjustment by providing switch circuits to each division of a photodetector between the photodetector and subtractor and performing a damping operation before a difference is calculated by the subtractor.
CONSTITUTION: A photodetector 20 divides reflected light from a medium into plural parts. A subtractor 21 obtains a servo error signal by performing subtraction on each divided output of the detector 20. An adder circuit 22 adds each divided output of the detector 20 to each other. Switch circuits 24 are switched to each other by the pulse output of a pulse generation circuit 23 and the switching output of the circuit 24 is averaged. In the case of writing time when the quantity of the reflected light increases is started, the output of the circuit 22 becomes larger than a target value and the duty ratio of the output pulse of the circuit 23 becomes smaller. Therefore, the damping factor of the circuit 24 increases and the servo gain can be made constant, because the circuit 22 is controlled so that the output of the circuit 22 can be dropped. In such a case, no bipolar circuit is required for the subtractor 21. Moreover, it is not necessary to generate an offset voltage, since digital operations by means of PWN control are performed.
MORITSUGU MASAHARU
NIMATA AKIO
