Variable-Bias Pixel Amplifier for Wider CMOS Sensor Output Range
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional CMOS image sensors face challenges in achieving high charge to voltage conversion ratios with minimal noise due to small size, high speed, and low power requirements, leading to reduced output signal ranges caused by reset switch charge injection and clock feed-through.
Innovation Solution
Implementing cascode amplifier circuits with capacitive transimpedance amplifiers and variable bias signals to improve the output signal range by reducing the impact of reset switch charge injection and clock feed-through.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If conventional amplifier circuits are used in small-sized pixel circuits, then integration and miniaturization are achieved, but output signal range is reduced due to reset switch charge injection and clock feed-through
Solution Approach 1:
The patent applies dynamics by making the bias signal variable rather than fixed. The bias signal is dynamically adjusted based on the operational state of the reset switch - when the reset switch is turned off, the bias signal transitions to a second value that compensates for charge injection effects. This dynamic adjustment allows the amplifier to adapt to changing conditions and maintain optimal performance in miniaturized pixel circuits.
Solution Approach 2:
The patent changes the bias parameter of the amplifier circuit from a constant value to a variable value. By changing the bias signal between a first value (when reset switch is on) and a second value (when reset switch is off), the patent compensates for the harmful effects of charge injection and clock feed-through. This parameter change enables the amplifier to maintain a larger output signal range despite the small pixel circuit size.
2Manufacturing precision
If amplifier circuits are miniaturized to meet high resolution requirements, then pixel density increases, but noise from backend readout circuitry increases and charge to voltage conversion ratio decreases
Solution Approach 1:
The patent implements feedback by monitoring the operational state of the reset switch and using this information to adjust the bias signal accordingly. The system detects when the reset switch transitions states and responds by changing the bias signal to compensate for the resulting effects. This feedback mechanism allows the amplifier to maintain high charge to voltage conversion ratio and low noise performance even in miniaturized pixel circuits with high density.
3Device complexity
If fixed bias signals are used in amplifier circuits, then circuit design is simplified, but output signal range is limited due to inability to compensate for charge injection and clock feed-through
Solution Approach 1:
The patent transitions from a static bias signal design to a dynamic one. The bias signal is no longer fixed but varies based on the reset switch state. This dynamic approach, while slightly increasing design complexity, effectively compensates for charge injection and clock feed-through effects, resulting in a significantly larger output signal range that justifies the additional design effort.
Data Source
AI summary
A pixel circuit includes a photodiode, and a transfer transistor coupled to the photodiode. A floating diffusion is coupled to the transfer transistor coupled to transfer image charge from the photodiode to the floating diffusion. An amplifier circuit includes an input coupled to the floating diffusion, an output coupled to generate an image data signal of the pixel circuit, and a variable bias terminal coupled to receive a variable bias signal. A reset switch is coupled between the output and input of the amplifier circuit to reset the amplifier circuit in response to a reset signal. A variable bias generator circuit is coupled to generate the variable bias signal in response to a reset signal to transition the variable bias signal from a first bias signal value to a second bias signal value in response to a transition of the reset signal from an active state to an inactive state.