Imaging Pixels with Feedback Loops for Noise Reduction
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Solution Overview
Problem
Conventional imaging sensing pixels are limited by the transfer efficiency and speed of transfer transistors, which restrict the performance of image sensors in converting incident light into electrical signals.
Innovation Solution
Incorporating a negative feedback loop in imaging pixels, where a common-source amplifier and feedback transistor clamp the voltage on the floating diffusion node, reducing reset noise and allowing transfer transistors to function as switches with minimal lag, enabling deeper photodiodes for improved light conversion efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional imaging sensing pixels are used with transfer transistors, then the structure is simple, but the charge transfer efficiency and speed are limited
Solution Approach 1:
The patent implements a feedback loop where the output of the differential amplifier is fed back to the gate of the transfer transistor. This feedback mechanism dynamically adjusts the transfer transistor's operation to maintain optimal charge transfer efficiency while managing the additional circuit complexity through systematic design
Solution Approach 2:
The pixel is divided into distinct functional blocks: photodiode, transfer transistor, floating diffusion node, and differential amplifier with feedback. This segmentation allows each component to be optimized independently for its specific function while maintaining overall system performance
2Speed
If transfer transistors are used to shift charge, then the pixel structure is manageable, but the transfer speed and efficiency are restricted
Solution Approach 1:
The feedback signal from the differential amplifier output is applied to the transfer transistor gate, creating a closed-loop system that accelerates charge transfer by dynamically controlling the transistor's conductive state based on real-time charge accumulation at the floating diffusion node
Solution Approach 2:
The feedback mechanism changes the operational parameters of the transfer transistor dynamically during the transfer process, adjusting gate voltage to optimize transfer speed while managing the complexity through standardized circuit design patterns
3Measurement precision
If deeper photodiodes are implemented, then light conversion efficiency improves, but transfer efficiency may be compromised without feedback loops
Solution Approach 1:
The feedback loop compensates for the increased charge transfer distance in deeper photodiodes by dynamically adjusting the transfer transistor operation, ensuring that even charges from greater depths are transferred efficiently to the floating diffusion node
Solution Approach 2:
The feedback mechanism prepares the transfer transistor in advance by pre-positioning the gate voltage based on expected charge arrival, optimizing the transfer path for charges generated at various depths within the extended photodiode region
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
The feedback loop reduces reset noise, enhances charge transfer efficiency, and allows for deeper photodiodes that can convert a larger proportion of incident light into electrical signals, improving image sensor performance without limiting transfer efficiency.
Implementation Method 1
Each pixel receives incident photons (light) and converts the photons into electrical signals
Data Source
AI summary
An imaging system may include an image sensor array formed from imaging pixels with feedback loops. Each imaging pixel may include an amplifier transistor that is controlled by a voltage on a floating diffusion node and may include a feedback transistor connected between the floating diffusion node and column readout circuitry. The amplifier transistor, together with a current source in the image sensor array, may form a common-source amplifier that inversely amplifies the voltage on the floating diffusion node and provides control signals to the feedback transistor. The common-source amplifier and the feedback transistor may create a feedback loop during image readout operations and during image reset operations that clamps the voltage on the floating diffusion node.