A new MOS imager using photodiode as current source

The author presents a MOS linear image sensor having a high-voltage gain amplifier in photodiode pixels. The only difference between the amplified MOS image (AMI) and this new device is the transfer gate added between the photodiode pixel and the source follower. In this structure, the photoinduced charge loss in video-line parasitic capacitance is compensated to improve the sensitivity. A high-voltage-gain amplifier is used to control the input voltage of the transfer gate so as to maintain an unchanged bias voltage of the photodiode and by operating the photodiode cell as a current source. In this device, the photoinduced charge is not divided by the photodiode capacitance C_d or the capacitive load C_t. Therefore, it is possible to enlarge the photodiode size, and the sensor has a large saturation exposure as well as a large photocurrent/dark-current ratio of about 30:1. The device operates on a single 5 V power supply. These features make the device suitable for applications in low light levels