Image Sensor Pixel Feedback Circuit for Dynamic Range and SN Ratio
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Solution Overview
Problem
Conventional solid-state imaging techniques face challenges in extending the dynamic range and improving signal-to-noise (SN) ratio.
Innovation Solution
A solid-state imaging apparatus is designed with a pixel circuit and a negative feedback circuit. The pixel circuit includes a photodiode, charge storage, transfer and amplification transistors, and storage capacitive elements. The negative feedback circuit feeds back a feedback signal to the charge storage, reducing reset noise and improving image quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a conventional negative feedback circuit is used to reduce reset noise, then reset noise is reduced, but the dynamic range cannot be extended and SN ratio cannot be improved
Solution Approach 1:
The pixel circuit is segmented into multiple functional blocks: photodiode for charge generation, charge storage for holding signal charge, multiple storage capacitive elements (first and second) for different gain modes, and transfer transistors for selective connection. This segmentation allows independent optimization of each block's function, enabling the circuit to switch between high-gain and low-gain modes to extend dynamic range while maintaining reset noise reduction capabilities.
Solution Approach 2:
The circuit employs dynamic switching between different operational modes through control transistors. The first storage capacitive element can be selectively connected or disconnected from the charge storage based on illumination conditions. In low-illuminance conditions, the first storage capacitive element is turned off to achieve high gain and improved SN ratio. In high-illuminance conditions, it is turned on to extend dynamic range. This dynamic reconfiguration resolves the contradiction between noise reduction and dynamic range extension.
2Adaptability or versatility
If the first storage capacitive element is turned on to extend dynamic range in high-illuminance conditions, then dynamic range is extended, but SN ratio deteriorates in low-illuminance conditions
Solution Approach 1:
The circuit changes its electrical parameters (capacitance configuration, gain level) based on illumination conditions. By controlling the first transistor to switch the first storage capacitive element between connected and disconnected states, the circuit adapts its total capacitance and gain characteristics. In low-illuminance conditions, the circuit operates in high-gain mode with the first storage capacitive element disconnected, optimizing SN ratio. In high-illuminance conditions, it switches to low-gain mode with the element connected, extending dynamic range. This parameter adaptation resolves the contradiction between dynamic range and SN ratio.
3Measurement precision
If the first storage capacitive element is turned off to improve SN ratio in low-illuminance conditions, then SN ratio is improved, but dynamic range is reduced
Solution Approach 1:
The circuit dynamically switches between high-gain and low-gain operational modes based on illumination detection. The control mechanism monitors illumination levels and automatically reconfigures the circuit topology by switching the first transistor. In low-illuminance conditions, the circuit transitions to high-gain mode with the first storage capacitive element disconnected, optimizing SN ratio. In high-illuminance conditions, it transitions to low-gain mode with the element connected, maintaining dynamic range. This dynamic adaptation allows the circuit to optimize for either SN ratio or dynamic range depending on conditions, resolving the contradiction.
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 proposed solution effectively extends the dynamic range and improves the SN ratio, enabling better low-illuminance and high-illuminance image quality while reducing reset noise.
Implementation Method 1
a photodiode; a charge storage; a transfer transistor that transfers a signal charge generated by the photodiode to the charge storage
Implementation Method 2
The negative feedback circuit negatively feeds back a feedback signal according to a reset output of the amplification transistor to the charge storage (FD0) via the first reset transistor
Data Source
AI summary
A solid-state imaging apparatus includes a pixel circuit and a negative feedback circuit. The pixel circuit includes: a photodiode; a charge storage that holds a signal charge generated by the photodiode; an amplification transistor that outputs a pixel signal corresponding to the signal charge in the charge storage; a first reset transistor that resets the charge storage; a first storage capacitive element for holding a signal charge; and a first transistor that controls the connection between the charge storage and the first storage capacitive element. The negative feedback circuit negatively feeds back a feedback signal corresponding to a reset output of the amplification transistor to the charge storage via the first reset transistor.