An Active Pixel Sensor with Built-in a Unity Gain Buffer

Document Type : Research Article


Faculty of Electrical Engineering, Urmia University of Technology, Urmia, Iran


This paper presents an active pixel sensor (APS) with a built-in readout circuit. A unity-gain buffer as a pixel readout circuit is used to read the signals of four adjacent pixels. Compared with the conventional four-transistor APS (4T-APS) which is connected to a CMOS source-follower readout circuit, the introduced circuit has higher accuracy, higher linearity, and fewer transistors. Due to the high linearity and accuracy of the pixel readout circuit, the proposed method can help to improve the final image quality of the sensor. For a fair comparison between the conventional 4T-APS (with source-follower readout circuit) and the proposed circuit, both circuits are designed with the same power consumption. Simulation results show that the proposed circuit is 16% more accurate, 11% higher fill factor, and 10 dB more linear than the conventional circuit. The total power consumption of the proposed circuit with a built-in buffer is almost 17 µW with a 1.8 V power supply and its layout size is 14.8µm×14.8µm. The total number of transistors used in this method to read the signals of four pinned-photodiodes is 11, whereas, in the conventional method, 16 transistors are required under the same circumstances. The proposed circuit is designed in 0.18µm CMOS technology.


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