Current Mode Pixel Readout Circuit with Feedback Stabilization
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Solution Overview
Problem
Analog pixels operating in voltage mode are slower than current mode due to higher photoresponse variations caused by local output device transconductance variations, leading to most pixels being limited to voltage mode operation.
Innovation Solution
A device comprising a pixel, a current source, and a readout circuit that samples output voltage during a first phase and output current during a second phase, with a current memory providing feedback current to maintain constant transconductance and reduce noise, allowing for improved radiation sensing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If pixels operate in current mode, then speed is improved, but photoresponse variations increase due to local transconductance variations
Solution Approach 1:
The patent implements a feedback mechanism where the output current of the pixel is fed back to the gate of the output transistor through a current memory element. This feedback stabilizes the transconductance by dynamically adjusting the gate voltage to compensate for local variations, thereby maintaining stable photoresponse while operating in fast current mode
Solution Approach 2:
The patent changes the operating parameters by switching between voltage mode and current mode operation based on the readout phase. During the first readout phase, the pixel operates in voltage mode with stable photoresponse, and during the second readout phase, it operates in current mode with fast speed, thus resolving the contradiction through parameter switching
2Reliability
If pixels operate in voltage mode, then photoresponse stability is improved, but operation speed decreases
Solution Approach 1:
The patent employs periodic switching between voltage mode and current mode operation in two distinct readout phases. The first phase uses voltage mode for stable photoresponse sampling, and the second phase uses current mode for fast signal readout, achieving both stability and speed through periodic mode switching
3Stability of the object's composition
If transconductance varies locally, then current mode operation becomes unstable, but voltage mode operation becomes slower
Solution Approach 1:
The feedback loop compensates for local transconductance variations by dynamically adjusting the gate voltage of the output transistor. This maintains stable transconductance during current mode operation, enabling fast readout without sacrificing stability
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
This approach enables faster and more stable radiation sensing by maintaining constant transconductance and reducing noise, effectively addressing the limitations of voltage mode operation in analog pixels.
Implementation Method 1
Pixels are used to sense incoming radiation... a radiation sensor of the pixel during a sensing period
Data Source
AI summary
A device comprising a pixel, a current source, and a readout circuit; wherein during a first readout phase of the pixel, the readout circuit is configured to sample a sampled output voltage of an output node of the pixel; and wherein an output current of the pixel is set by the current source; wherein between the first readout phase of the pixel and a second readout phase of the pixel, the pixel is configured to change the output current to provide a second output current, wherein the change of the output current is responsive to radiation sensed by a radiation sensor of the pixel during a sensing period; wherein during the second readout phase of the pixel the readout circuit is configured to sample the second output current while providing the sampled output voltage to the output node of the pixel; wherein during the first readout phase of the pixel and the second readout phase of the pixel a drain source voltage of an output transistor of the pixel is maintained constant; and wherein the output transistor is configured to generate the second output current.