| 摘要 |
PURPOSE:To improve the minimum measuring level of incident light power and to obtain a wide dynamic range by impressing reverse bias voltage which is zero or minute at a region having small incident light power and large at a region having large power to a photo diode. CONSTITUTION:When the incident light is given to the photo diode 1, a light detecting current I0 flows, a voltage VL corresponding to the current I0 is generated across a load resistance RL and a positive output voltage V0 corresponding thereto is obtained. The output voltage V0 is inputted also to a voltage generating circuit 5 for reverse bias and an output voltage V1 for reverse bias is obtained at the output terminal of an operational amplifier 6 for reverse bias but the diode 8 is kept turned off until the output voltage V0 rises to a prescribed value. Thus, the output voltage V1 is kept nearly zero and the reverse bias voltage of the photo diode 1 is kept also nearly zero. The forward series resistance value of the diode 8 is decreased with the increase in the incident light power, the output voltage V1 is increased and the reverse bias of the photo diode 1 is made deep. |
