Active Reset Feedback Amplifier for Stacked Image Sensor Noise Reduction
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Solution Overview
Problem
The shrinking pixel size in CMOS image sensors is limited by the need for two storage capacitors for reset and signal voltage in Correlated Double Sampling (CDS), which increases layout size and hinders noise reduction, particularly fixed pattern noise (FPN).
Innovation Solution
A method and apparatus that eliminate one storage capacitor by using a column feedback loop and active reset circuit, allowing for a single storage capacitor while maintaining noise reduction through differential amplification and shared reset drive voltages, enabling smaller pixel sizes and reduced FPN.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If two storage capacitors are used for reset and signal voltage in CDS, then noise reduction is achieved, but pixel size cannot be reduced further
Solution Approach 1:
The patent merges the reset signal storage function with the signal path by using the same storage capacitor for both reset voltage (SHR) and signal voltage (SHS) sampling. The feedback amplifier shares this capacitor between reset and signal operations, eliminating the need for separate storage capacitors while maintaining CDS noise reduction capability through differential amplification of the shared capacitor's voltage differences.
2Reliability
If large layout size is used for storage capacitors, then kTC thermal noise is reduced, but pixel shrinkage is limited
Solution Approach 1:
The storage capacitor is designed to serve multiple functions: storing reset voltage, storing signal voltage, and participating in the feedback loop for both reset and signal amplification. This multi-functional design allows a single capacitor to replace what would traditionally require two separate capacitors, reducing the total layout area while maintaining sufficient capacitance value for kTC noise suppression.
3Area of moving object
If one storage capacitor is used, then pixel size can be reduced, but maintaining noise reduction becomes difficult
Solution Approach 1:
The patent implements a feedback amplifier that continuously monitors and amplifies the voltage on the shared storage capacitor. During reset phase, the feedback amplifier amplifies the reset voltage; during signal phase, it amplifies the signal voltage. This feedback mechanism ensures that even with a single smaller capacitor, the noise reduction performance is maintained through precise differential amplification of the voltage differences between reset and signal states.
Data Source
AI summary
An image sensor comprises a pixel array of pixel cells. A pixel cell comprises a photodiode, a reset transistor, a transfer transistor, at least one source follower transistor, a sample and hold circuit, an active reset transistor, and a readout transistor. A readout circuitry reads out image data from each columns of pixel cells. A column differential amplifier in the readout circuitry feeds back a column reset drive voltage to each pixel cells arranged in the same column. Signal data of each pixel cells in the same column are read out globally when all the active reset transistors are switched off. Determined by switching configurations of each active reset transistors of pixel cells in the same column, noise data of each pixel cells in the same column are read out either globally or row-by-row. Final image data is achieved by applying the method of correlated double sampling (CDS).