Column-Level Amplifier Automatic Gain Selection for CMOS Imager Noise
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Solution Overview
Problem
CMOS imagers face challenges in increasing intrascene dynamic range and signal-to-noise ratio, particularly for higher signal levels, which can result in increased noise and signal clipping due to limited amplification capabilities.
Innovation Solution
The implementation of pixel-wise automatic gain selection in the column circuitry, allowing for analog gain application on a per-pixel basis, either with unity gain or greater than unity gain, using a comparator to decide between direct sampling of the pixel signal or sampling after amplification in a column level amplifier.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a constant current source provides biasing current for all pixels on the same column, then the circuit can operate with shared column-level amplification, but the signal-to-noise ratio deteriorates for higher signal levels due to limited amplification capabilities
Solution Approach 1:
The patent divides the column-level amplification into two separate paths: a first column amplifier for low signal levels and a second column amplifier for high signal levels. This segmentation allows each amplifier to be optimized for its specific signal range, improving the signal-to-noise ratio without requiring a single complex amplifier to handle all signal levels
Solution Approach 2:
The patent introduces dynamic switching between different amplification paths based on signal level detection. A detector monitors the pixel output signal and automatically selects whether to route the signal through the first or second amplifier, enabling adaptive amplification that maintains optimal signal-to-noise ratio across varying lighting conditions
2Measurement precision
If analog gain is applied at the column level for all pixels, then amplification is achieved, but signal clipping occurs for higher signal levels
Solution Approach 1:
The patent segments the amplification function into two distinct amplifiers with different gain characteristics. The first amplifier provides high gain for low signal levels, while the second amplifier provides lower gain for high signal levels, preventing clipping by matching the amplifier gain to the signal level
Solution Approach 2:
The patent employs a detector that monitors the pixel output signal level and provides feedback control for selecting the appropriate amplifier path. This feedback mechanism ensures that the amplification is automatically adjusted to prevent signal clipping while maintaining optimal signal-to-noise ratio
3Adaptability or versatility
If pixel-wise automatic gain selection is implemented, then intrascene dynamic range is improved, but device complexity increases
Solution Approach 1:
The patent implements pixel-wise automatic gain selection by segmenting the column circuit into multiple functional blocks: a detector for signal level detection, switching circuitry for signal routing, and multiple amplifiers for different gain levels. This modular segmentation enables flexible per-pixel gain control while keeping each individual block relatively simple
Solution Approach 2:
The patent creates a universal column circuit structure that can handle multiple signal levels and lighting conditions through a single integrated design. The multi-functional column circuit uses shared components (detector, switches, capacitors) that serve different purposes depending on the signal level, reducing overall complexity while maintaining adaptability
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 approach enhances the signal-to-noise ratio by enabling efficient amplification without signal clipping, achieving true correlated double sampling and automatic gain selection based on lighting conditions, thereby improving the dynamic range and noise performance of the imager.
Implementation Method 1
a focal plane array of pixel circuits, each one of the pixels including a photosensor, for example, a photogate, photoconductor or a photodiode overlying a substrate for accumulating photo-generated charge
Implementation Method 2
The charge at the storage region is typically converted to a pixel output voltage by a source follower output transistor
Implementation Method 3
The source follower transistor 28 converts the stored charge at the floating diffusion region FD into an electrical output voltage signal VOPIX
Data Source
AI summary
An imager architecture that utilizes column sampling circuitry that can support pixel-wise automatic gain selection (AGS). The column sampling circuitry samples the pixel output signals directly (e.g., with unity gain) or after amplification in a column level amplifier while supporting correlated double sampling (CDS) in both situations.