| 摘要 |
PURPOSE:To process the output signal of a photodetector extending over a wide dynamic range and to improve the S/N of a read signal by providing a gain switching means that switches the gain of an amplifier to plural stages and a control circuit. CONSTITUTION:The output signal S of a photomultiplier 20 is sent to a logarithmic amplifier 23 that consists of an amplifier 21 and a logarithmic computing element 22 and an amplification and a logarithmic conversion are performed on it. At such a case, at the feedback line of the amplifier 21, feedback resistors 26 and 27 and a switch 28 are provided constituting the gain switching means that switches the gain of the amplifier 21. Namely, the gain of the amplifier 21, through a comparison circuit 29, is switched comparatively to be low when the level of the signal S is comparatively high and is switched comparatively to be high when that of the signal S is comparatively low. Thus, by providing the gain switching means which consists of resistors 26 and 27 and the switch 28, it is possible to process the output signal of the photodetector extending over the wide dynamic range and to improve the S/N of the read signal.
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