Pixel Circuit Buffering Voltage Swing Limitations
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Solution Overview
Problem
The existing pixel circuits in CMOS image sensors face limitations in signal swings due to voltage differences between input and output signals, exacerbated by the body effect in source followers, which reduces the overall performance.
Innovation Solution
A novel pixel circuit design incorporating a photodiode, buffer circuit, capacitors, and switches that bypass the source follower, utilizing a switch-capacitor amplifier to enhance signal swings and mitigate the body effect, allowing for improved read-out signal voltage levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a source follower is used to output read-out signals, then the circuit structure is simple, but the voltage swing of read-out signals is limited
Solution Approach 1:
The patent extracts the problematic source follower component from the pixel circuit and replaces it with a buffer circuit. This removal eliminates the voltage swing limitation inherent in source followers while maintaining the essential signal buffering function, thereby resolving the contradiction between structural simplicity and voltage swing capability
Solution Approach 2:
The patent changes the operational parameters of the buffering stage by using a buffer circuit instead of a source follower. This parameter change enables the read-out signals to achieve larger voltage swings (from limited swings to nearly full rail-to-rail swings) while keeping the overall circuit complexity manageable through careful design
2Ease of manufacture
If a source follower is used, then the circuit implementation is straightforward, but body effect causes threshold voltage variation
Solution Approach 1:
The patent removes the source follower from the circuit, eliminating the body effect problem that causes threshold voltage variation. The buffer circuit replacement does not suffer from this effect, thereby improving threshold voltage stability and signal reliability while maintaining ease of implementation
Solution Approach 2:
The patent replaces the problematic source follower with a buffer circuit implementation that achieves the same functional goal without the detrimental body effect. This substitution uses standard transistor configurations that are equally easy to manufacture but provide stable threshold voltages, improving reliability without increasing complexity
3Adaptability or versatility
If a source follower is used to buffer signals, then the circuit design is conventional, but signal swings are reduced
Solution Approach 1:
The patent extracts the signal buffering function from the source follower and implements it through a dedicated buffer circuit. This separation allows the buffering operation to be optimized independently, achieving full voltage swing capability while maintaining the conventional and flexible circuit design approach
Solution Approach 2:
The patent changes the buffering mechanism from source follower operation to buffer circuit operation. This parameter change transforms the signal swing characteristic from limited to maximized, while the buffer circuit maintains the same level of design flexibility and adaptability to different pixel circuit configurations
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 design increases the voltage swings of read-out signals, thereby enhancing the overall performance of the pixel circuit without the drawbacks of source followers, such as voltage swing limitations and body effect issues.
Implementation Method 1
The photodiode is configured to accumulate charges in response to incident radiation, to generate a photodiode signal
Data Source
AI summary
A pixel circuit is provided, where the pixel circuit comprises a photodiode, a buffer circuit, a first capacitor, a first switch, a second switch and a third switch. The photodiode is configured to accumulate charges in response to incident radiation, to generate a photodiode signal. The buffer circuit is configured to output at least one read-out signal, wherein an input terminal of the buffer circuit is coupled to a specific node. The first capacitor is coupled between a control voltage terminal of the pixel circuit and the specific node. The first switch is coupled between the photodiode and the specific node. The second switch is coupled between the input terminal of the buffer circuit and an output terminal of the buffer circuit. The third switch is coupled between the output terminal of the buffer circuit and a read-out terminal of the pixel circuit.