CMOS Ramp Signal Circuit for Stable Voltage Under Transistor Mismatch
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Solution Overview
Problem
CMOS image sensors face challenges in generating accurate and stable ramp voltages due to transistor characteristic mismatches caused by manufacturing process variations, affecting the accuracy and stability of the ramp signal generation.
Innovation Solution
A ramp signal generation device comprising a sampling circuit, current maintaining circuit, current transferring circuit, and current-to-voltage converter is used to sample, maintain, and convert the ramp current into a stable and accurate ramp voltage, employing a capacitor and switching circuit to store and transfer the current, thereby stabilizing the output voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If transistors are used to copy and transfer ramp current, then the ramp signal can be generated, but transistor characteristic mismatches cause instability and inaccuracy in the ramp voltage
Solution Approach 1:
The patent introduces a capacitor as an intermediary element to store the ramp voltage. The capacitor decouples the ramp current generation (through mismatched transistors) from the ramp voltage output, allowing the voltage to be held stable regardless of transistor variations. This mediator absorbs the instability caused by manufacturing mismatches.
Solution Approach 2:
The patent creates a copied version of the ramp signal through the capacitor, which stores the voltage corresponding to the ramp current. This copied voltage signal is then used for comparison, separating the function from the original transistor-based current generation and eliminating the impact of transistor mismatches on the final voltage accuracy.
2Adaptability or versatility
If current steering digital-to-analog conversion is used, then gain adjustment is enabled, but transistor mismatches still affect ramp voltage accuracy
Solution Approach 1:
The capacitor serves as a mediator between the adjustable current steering DAC and the voltage output. While the DAC provides gain adjustment through programmable current control, the capacitor ensures that the resulting voltage remains stable and accurate by storing and holding the voltage level independent of subsequent transistor variations in the transfer path.
3Ease of operation
If multiple transistors are used in the ramp signal generation path, then current copying and transfer is achieved, but process distribution causes characteristic variations
Solution Approach 1:
The patent extracts the voltage storage and holding function into a separate capacitor element, removing it from the transistor-based current transfer path. This extraction allows the transistors to perform their current copying function while the capacitor independently maintains voltage stability, separating the two functions to eliminate the impact of transistor variations on voltage consistency.
Data Source
AI summary
A ramp signal generation device includes a sampling circuit suitable for sampling a ramp current and storing a voltage corresponding to the sampled ramp current; a current maintaining circuit suitable for maintaining the ramp current; a current transferring circuit suitable for transferring a current corresponding to the voltage stored on the sampling circuit; and a current-to-voltage converter suitable for converting and generating a ramp voltage corresponding to the current transferred from the current transferring circuit.