Pixel Circuit Negative Feedback Loop for Charge Transfer
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional CMOS imagers face limitations in full-well charge capacity and image lag due to the trade-off between charge storage capacity and dynamic range, especially when the full-well capacity of the photosensor exceeds the capacity of the floating diffusion node, leading to charge sharing and increased fixed pattern noise.
Innovation Solution
The implementation of a pixel circuit with a negative feedback control loop, utilizing an amplifier and a capacitance element that creates a capacitance divider to fix the voltage of the charge storage node, allowing the transfer transistor to act as a switch and reducing dependence on storage region capacitance, thereby increasing full-well charge capacity while minimizing image lag and noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the full-well charge capacity of the photosensor is increased, then the charge storage capacity is improved, but image lag increases due to charge sharing when the photosensor capacity exceeds the floating diffusion node capacity
Solution Approach 1:
The patent introduces a transfer transistor as an intermediary component between the photosensor and the floating diffusion node. This transfer transistor acts as a controlled mediator that regulates charge transfer, ensuring complete charge evacuation from the photosensor to the floating diffusion node, thereby preventing charge sharing and image lag while allowing high full-well charge capacity
Solution Approach 2:
The patent implements a feedback mechanism through the transfer control signal that monitors and controls the charge transfer process. The feedback ensures that charge transfer is completed fully before the next integration cycle begins, preventing residual charge from causing image lag in subsequent frames
2Measurement precision
If the floating diffusion node capacitance is reduced to increase dynamic range, then the measurement precision is improved, but the charge holding capacity decreases leading to charge sharing
Solution Approach 1:
The patent segments the charge storage function into two distinct components: the photosensor maintains high charge holding capacity for maximum dynamic range, while the floating diffusion node serves as a controlled transfer destination with precisely timed charge acceptance. The transfer transistor coordinates this segmentation, allowing the photosensor to accumulate full-well charge without being constrained by the smaller floating diffusion node capacity
Solution Approach 2:
The patent applies preliminary action by resetting the floating diffusion node to a known voltage state before each charge transfer cycle. This preliminary reset ensures the floating diffusion node is ready to accept the full charge signal from the photosensor, preventing charge sharing and maintaining measurement precision across the full dynamic range
3Device complexity
If multiple pixels share common readout circuitry, then the device complexity is reduced, but the charge transfer efficiency decreases due to capacitive loading
Solution Approach 1:
The patent ensures continuity of useful action by maintaining the transfer transistor in a controlled state that continuously manages charge flow during the transfer phase. This continuous controlled transfer, coordinated across multiple pixels sharing common readout circuitry, maintains charge transfer efficiency despite capacitive loading by ensuring complete and synchronized charge evacuation from each photosensor
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution enables higher conversion gain and reduced readout noise, allowing for accurate signal representation near full-well capacity without degrading the dynamic range, and enables efficient sharing of common output circuitry among multiple pixels without capacitance limitations.
Implementation Method 1
each one of the pixels including at least one photosensor overlying a substrate for accumulating photo-generated charge in the underlying portion of the substrate
Data Source
AI summary
A pixel circuit having improved charge transfer including an amplifier having an input node electrically connected to a charge storage node of the pixel circuit, and a negative feedback control loop having a capacitance element electrically connected between the input node and an output node of said amplifier.