Image Sensor Readout Circuit Noise Reduction
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Solution Overview
Problem
High-resolution image sensors above one Mega-pixel face challenges in removing horizontal noise caused by increased parasitic capacitances between load transistors, which conventional methods struggle to address without compromising dynamic range and optic characteristics.
Innovation Solution
The image sensor adjusts the size ratio of active transistors and load transistors in the readout circuit, with the active transistor being larger, to stabilize bias voltage and reduce horizontal noise without affecting the dynamic range or optic characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the bias current is increased to reduce horizontal noise, then the horizontal noise is reduced, but the dynamic range of the pixels decreases and optic characteristics deteriorate
Solution Approach 1:
The patent changes the size parameter of the active transistor (increasing its width) to alter the current distribution in the readout circuit. This parameter change allows the active transistor to source sufficient bias current to reduce horizontal noise while preventing excessive current from degrading the dynamic range, thus resolving the contradiction between noise reduction and dynamic range preservation
2Measurement precision
If the resolution of the image sensor is increased to one Mega-pixel or higher, then the image quality is improved, but horizontal noise occurs due to increased parasitic capacitances
Solution Approach 1:
The patent modifies the size parameter of the active transistor in the readout circuit to compensate for the increased parasitic capacitances resulting from higher resolution. By increasing the active transistor size, sufficient bias current is maintained despite the higher capacitive loads from additional pixels, thereby preventing horizontal noise while preserving the high resolution benefit
Data Source
AI summary
Disclosed herein is an image sensor for removing a horizontal noise. The image sensor includes a pixel array including a plurality of unit pixels located at every row or column; an analog bus located at every row or column, for transferring output signals of the pixel array; and a readout circuit for reading the output signals of the pixel array loaded on the analog bus, wherein the readout circuit includes: a plurality of first transistors having a drain connected to the column analog bus and a source connected to a supply voltage; and a second transistor having a drain connected to a gate of the second transistor and gates of the first transistors, and a source connected to the supply voltage, wherein, a size of the second transistor is larger than a size of the first transistor.