| 摘要 |
PURPOSE:To enable accurate and stable tracking control by finding the difference between two light receiving means, detecting DC offset in the input/output system of the two light receiving means from input signals and output signals of a differential amplifier, supplying the detection signal to the differential amplifer and canceling the offset. CONSTITUTION:For instance, if there is a difference in sensitivity between photodiodes 21 and 31, the DC level of output of amplifiers 4 and 5, which is intrinsically fixed, fluctuates. Consequently, a tracking error signal on which a DC offset signal d is superposed appears in an output of a differential ampli fier 6. An output of the differential amplifier 6 is inputted to a low-pass filter 7. As a time constant of a resistance 71 and a capacitor 72 of the low-pass filter 7 is selected to cut off an intrinsic tracking error signal component which is an AC component, only a DC offset signal (d) is outputted. As the signal d is inputted to an invertion terminal of an operational amplifier 64 through a resistance 66, a tracking error signal in which the DC offset is canceled is outputted from the differential amplifier 6.
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