| 摘要 |
PROBLEM TO BE SOLVED: To surely use optical element signals by receiving an application using the small difference of light intensity, that is the light of a small level difference. SOLUTION: A servo circuit for stabilizing a voltage level in the base node of an optical element is provided and the ratio of a light receiving area to the total area of navigation sensors 24 and 26 is raised. The servo circuit is formed by three MOS transistors 50, 52 and 54 and the three transistors 50, 52 and 54 form a bias point amplifier provided with a common gate stage for the output of a phototransistor 36. In order to appropriately transmit a current generated in an optical element circuit 40 to an integration capacitor 30, the inverse voltage of a photodiode 34 is required to be maintained at an almost fixed level. When the voltage of the base node 56 is changed, an optical current is at least partially consumed in the charge and discharge of a diode capacitance 38 without becoming the current amplified by a substrate PNP transistor 36.
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