Pixel Sensing Circuit Threshold Voltage Compensation
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Solution Overview
Problem
CMOS image sensors face issues with non-uniform output from source follower transistors due to drifting threshold voltages, leading to inconsistent conversion of optical signals into electrical signals over time.
Innovation Solution
A pixel sensing circuit with a compensation and pulling-up sub-circuit that generates a compensation voltage based on the threshold voltage of the conversion transistor, allowing it to operate in a saturated state, thereby making the output current independent of the threshold voltage and ensuring consistent signal conversion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If source follower transistors are used for signal conversion, then the circuit can be easily integrated into peripheral systems, but the threshold voltage drifts over time causing non-uniform output
Solution Approach 1:
The patent applies preliminary action by measuring and storing the threshold voltage of each conversion transistor before it drifts and affects output uniformity. The threshold voltage is measured during an initialization phase and stored in a storage circuit, then used to generate compensation voltages that pre-correct for expected threshold variations, ensuring uniform output throughout operation.
Solution Approach 2:
The patent implements feedback by using the measured threshold voltage to generate a compensation voltage that is fed back to the signal collection point. This compensation voltage adjusts the operating point of the conversion transistor to counteract threshold voltage drift, maintaining consistent current output despite variations in transistor characteristics over time.
2Reliability
If compensation voltage is applied to correct threshold voltage drift, then output uniformity is improved, but circuit complexity increases
Solution Approach 1:
The patent merges multiple functions into existing circuit elements. The storage circuit stores both the measured threshold voltage and the generated compensation voltage, and the same control circuits used for normal operation also manage the compensation process. This integration minimizes additional hardware while achieving threshold compensation.
Solution Approach 2:
The patent introduces a compensation voltage as an intermediary signal that mediates between the threshold voltage measurement and the final current output. This compensation voltage is added to the signal collection point voltage, serving as a corrective intermediary that adjusts the effective gate-source voltage to maintain uniform output current despite threshold variations.
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
The solution ensures that the current output from the conversion transistor is consistent and related only to the voltage variation at the signal collection point, independent of the threshold voltage, thereby alleviating the issue of non-uniform outputs from source follower transistors.
Implementation Method 1
a photoelectric conversion sub-circuit electrically coupled to a signal collection point and configured to collect an incident light, and generate an electrical signal based on the incident light collected
Data Source
AI summary
The embodiments of the present disclosure provide a pixel sensing circuit. The photoelectric conversion sub-circuit is configured to collect an incident light, and generate an electrical signal based on the incident light collected, so as to charge the signal collection point. The resetting sub-circuit is configured to write a first power supply voltage from the first power supply terminal to the signal collection point. The compensation and pulling-up sub-circuit is configured to: obtain a threshold voltage of the conversion transistor; generate a compensation voltage according to the threshold voltage; write the compensation voltage to the signal collection point; pull up the voltage at the signal collection point. The selection sub-circuit is configured to control the connection between the first electrode of the conversion transistor and an image processor. The conversion transistor is configured to output the current signal according to the voltage at the signal collection point.