| 摘要 |
PURPOSE:To constitute the simple light receiving circuit which is suitable for the operation at high temperatures and for sensing of very small amount of light by eliminating a dark current due to a light receiving element which does not receive the light from the current sensed by a light receiving element which actually receives the light. CONSTITUTION:The dark current of the light receiving element PT1, whic does not receive the light and which is used for cancelling the dark current, is the same as the dark current of the light receiving element PT2 which actually receives the light. The emitter currents ID and I3 of transistor Tr1 and Tr2 which are connected to both light receiving elements are adjusted to be equal, and a current mirror circuit is constituted. In said light receiving circuit, since the base current Ib=IPT2- I3 (where IPT2 is light current of PT2 + dark current) of Tr3 in the case the light from LED is inputted to PT2 is ID=I1 (comparison reference current)+IPT1 (dark current of PT1), Ib=(IPT2-IPT1)-I1 is obtaind. Therefore, the effect of the dark current is eliminated. Thus, the erroneous operation at high temperature can be eliminated, and the vary small amount of light can be sensed at a normal temperature. |
