CMOS Pixel Signal Transfer Device with Conversion Gain Adjustment
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Solution Overview
Problem
Conventional CMOS image sensors face challenges in directly converting small pixel signals due to high parasitic capacitance in column lines, leading to low conversion gain and sensitivity to process variations, especially when using small feedback capacitors.
Innovation Solution
A pixel signal transfer device with a transfer block, correction block, and conversion gain adjusting block that includes a transfer capacitor, amplification transistor, and resistors, allowing for stable adjustment of conversion gain by resetting and initializing node voltages and using resistors to correct and amplify pixel output voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a feedback capacitor with very small capacitance (1 to 2 fF) is used to increase conversion gain, then the conversion gain increases, but the device becomes very sensitive and vulnerable to process variation
Solution Approach 1:
The patent changes the parameter used for conversion gain adjustment from capacitor capacitance to transistor channel width. Instead of varying the small capacitance value (1-2 fF) of the feedback capacitor, the invention varies the width of the amplification transistor's channel, which has much lower sensitivity to process variation and can be precisely controlled during manufacturing.
Solution Approach 2:
The patent creates a copy of the conversion gain adjustment function using a different physical mechanism. Rather than directly adjusting the feedback capacitor's capacitance, it uses the amplification transistor's channel width as a proxy control parameter, which achieves the same functional effect (adjusting conversion gain) but with much better process stability.
2Area of stationary object
If the pixel signal is directly transferred to an analog-digital converting device, then the light receiving area is maximized, but the small pixel signal cannot be directly converted due to high parasitic capacitance in column lines
Solution Approach 1:
The patent introduces an intermediary amplification stage between the pixel and the analog-digital converting device. The amplification transistor acts as a mediator that boosts the small pixel signal before it reaches the converter, making the signal large enough to be accurately converted despite the presence of parasitic capacitance in the column lines.
Solution Approach 2:
The patent segments the signal processing function into distinct stages: signal transfer, signal amplification, and analog-digital conversion. By separating the amplification function into its own stage with dedicated transistors, the system can handle small pixel signals effectively without compromising the light receiving area of the photodiode.
3Device complexity
If multiple pixels are coupled to one column line, then the device complexity is reduced, but the conversion gain becomes very low and difficult to control
Solution Approach 1:
The patent makes the amplification transistor serve multiple functions simultaneously: it acts as a buffer to drive the column line, provides signal amplification to maintain conversion gain, and enables precise control of the conversion gain through its channel width. This multi-functional design allows multiple pixels to share a column line while maintaining adequate conversion gain.
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 easy adjustment of conversion gain while being insensitive to process variations, improving the stability and accuracy of pixel signal transfer in CMOS image sensors.
Implementation Method 1
a photodiode may be implemented to have a maximum size, and the pixel may be less sensitive to process variation because light received by the photodiode may be converted into a charge
Data Source
AI summary
A pixel signal transfer device includes a transfer block suitable for transferring a pixel output voltage according to an amount of a charge generated from a pixel; a correction block suitable for correcting the pixel output voltage using a threshold voltage of an amplification transistor; and a conversion gain adjusting block including the amplification transistor, the conversion gain adjusting block being suitable for adjusting a conversion gain of the corrected pixel output voltage outputted from the correction block.