Source Follower Gain Measurement in CMOS Image Sensor Arrays
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Solution Overview
Problem
There is no direct method to accurately measure the gain of source follower transistors in CMOS imagers, leading to the use of approximate values in design and comparison of imager outputs.
Innovation Solution
A variable voltage supply is introduced to selectively vary the voltage applied to a subset of pixels in the imager array, allowing for the measurement of source follower gain by altering the voltage during the readout process and using differential amplification to calculate the gain based on the voltage changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If no direct measurement method is used, then design and comparison processes are simpler, but measurement precision of source follower gain is insufficient
Solution Approach 1:
The measurement method segments the pixel array into two groups: pixels with normal source follower transistors and pixels with modified source follower transistors having different channel widths. This segmentation allows direct comparison and measurement of gain differences without requiring complex external measurement equipment, thereby improving measurement precision while avoiding excessive system complexity
Solution Approach 2:
The invention creates a copy of the pixel array with modified source follower transistors (different channel widths) to serve as a reference for measurement. By comparing the output signals from the original pixels and the copied pixels with known gain differences, the measurement system can directly determine source follower gain without complex external apparatus
2Reliability
If approximate gain values are used, then design and manufacturing processes are simpler and faster, but reliability of imager output comparison deteriorates
Solution Approach 1:
The measurement method performs preliminary actions by incorporating pixels with pre-modified source follower transistors (different channel widths) into the pixel array during manufacturing. This preliminary preparation enables direct gain measurement during normal operation without requiring additional time-consuming external measurement steps, thereby improving reliability without significant time loss
Solution Approach 2:
The pixel array serves itself for gain measurement by using its own pixels with modified source followers as reference elements. The measurement is performed using the imager's existing readout circuitry and processing capabilities, eliminating the need for external measurement equipment and additional measurement time, thus improving reliability while maintaining efficient design and manufacturing processes
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
Enables precise measurement of source follower transistor gain, improving the accuracy of conversion gain and floating diffusion responsivity calculations in CMOS imagers.
Implementation Method 1
A variable voltage supply is introduced to selectively vary the voltage applied to a subset of pixels in the imager array, allowing for the measurement of source follower gain by altering the voltage during the readout process
Implementation Method 2
using differential amplification to calculate the gain based on the voltage changes
Data Source
AI summary
Disclosed embodiments provide a method and apparatus for measuring the gain of output transistors of pixels in an imager device. Source/drain terminals of the output transistor and a reset transistor are driven with various input voltages to generate pixel output voltages. The slope of a line representing the relationship between the output voltages and the input voltages is determined. A component of the slope corresponding to gain not caused by the output transistor is removed from the slope to determine the gain of the output transistor.