Variable Amplifier Buffer Stage for CMOS Sensor Noise and Bandwidth
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Solution Overview
Problem
CMOS sensors face challenges in increasing read rate while suppressing fixed-pattern noise, as higher gain settings narrow the column amplifier band, degrading response characteristics and increasing noise variability.
Innovation Solution
A photoelectric conversion device with a variable amplifier stage and a buffer stage, where the buffer stage drives the holding capacitor, allowing for uniform bandwidth regardless of amplification factor, and includes a feedback amplifier circuit or voltage follower to reduce fixed-pattern noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the gain of the column amplifier is increased to suppress random noise and fixed-pattern noise, then the noise suppression performance is improved, but the bandwidth of the column amplifier becomes narrower, degrading the response characteristic
Solution Approach 1:
The amplifier is divided into two independent stages: a variable gain amplifier stage for noise suppression and a buffer stage for maintaining bandwidth. This segmentation allows each stage to optimize its function without compromising the other, resolving the contradiction between gain and bandwidth.
Solution Approach 2:
The buffer stage acts as an intermediary between the variable gain amplifier and the holding capacitor. It isolates the gain stage from the capacitive load, preventing bandwidth degradation while maintaining the noise suppression benefits of high gain.
2Speed
If the holding capacitor capacitance is decreased to widen the amplifier bandwidth, then the response characteristic is improved, but the relative variations between capacitance values increase, increasing fixed-pattern noise
Solution Approach 1:
The system is segmented into a gain stage and a buffer stage, allowing the holding capacitor to be optimized for low fixed-pattern noise (larger capacitance) while the buffer stage maintains the bandwidth response characteristic.
3Productivity
If the read rate is increased to maintain frame rate with higher pixel count, then the productivity is improved, but the horizontal blanking period must be shortened, making it difficult to satisfy both capacitance precision and response characteristic requirements
Solution Approach 1:
By segmenting the amplifier into gain and buffer stages, the system can use larger holding capacitors for precision while maintaining fast response through the buffer stage, enabling higher read rates without sacrificing precision.
Solution Approach 2:
The buffer stage changes the effective output impedance parameter, allowing the holding capacitor to have larger capacitance value while the amplifier maintains its bandwidth characteristics, thus satisfying both precision and speed requirements at high read rates.
Data Source
AI summary
A photoelectric conversion device includes a pixel output line, a pixel which outputs a signal to the pixel output line, an amplifier unit which amplifies the signal output to the pixel output line, and a holding capacitor which holds the signal output from the amplifier unit. The photoelectric conversion device outputs a pixel signal based on the signal held by the holding capacitor. The amplifier unit includes a variable amplifier stage which amplifies a signal output to the pixel output line at a gain selected from a plurality of gains, and a buffer stage which amplifies the signal output from the variable amplifier stage, the amplified signal being held by the holding capacitor to hold the signal.