Image Acquisition Circuit With Back-Gate Feedback for Reset Noise
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Solution Overview
Problem
Electronic circuits for image sensors face significant noise issues due to photodiode reset noise, which is typically ten times higher than other noise sources, especially in pixel architectures like the '3T' type and Capacitive Transimpedance Amplifier (CTIA), affecting the accuracy of illumination measurement.
Innovation Solution
The implementation of an electronic circuit with a servo circuit that adjusts the rear gate voltage of the amplifier transistor based on the output voltage and a reference voltage, using a differential amplifier to correct the noise, and incorporating a sampling capacitor to store correction values, thereby reducing the impact of thermal noise associated with the photodiode capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a photodiode reset is performed to measure illumination level, then the voltage across the photodiode can be measured, but random noise with standard deviation sqrt(kT/C) is introduced
Solution Approach 1:
The patent applies feedback by using a control circuit that continuously monitors the output voltage of the image acquisition cell and adjusts the back gate voltage of the amplifier transistor accordingly. The control circuit compares the output voltage with a reference voltage and modifies the back gate voltage to compensate for noise, thereby reducing the impact of reset noise on measurement precision.
Solution Approach 2:
The patent changes the operating parameters of the amplifier transistor by adjusting its back gate voltage. This parameter change allows dynamic control of the transistor's threshold voltage, enabling the system to optimize performance and reduce noise impact during different operating conditions without changing the fundamental circuit architecture.
2Device complexity
If pixel architectures without transfer gates (3T or CTIA) are used, then device complexity is reduced, but noise induced by photodiode reset increases by a factor of ten
Solution Approach 1:
The control circuit implements feedback control by monitoring the output voltage and adjusting the back gate voltage of the amplifier transistor. This feedback mechanism specifically targets and reduces the dominant reset noise in simplified pixel architectures without requiring complex additional circuitry like transfer gates.
Solution Approach 2:
The back gate voltage acts as an intermediary control parameter that mediates between the photodiode output and the final measurement. By controlling the back gate voltage, the system can indirectly adjust the amplifier's behavior to compensate for reset noise, providing a effective noise reduction mechanism in simple pixel architectures.
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 solution effectively reduces the standard deviation of noise associated with the photodiode capacitance, improving the signal-to-noise ratio and enhancing the accuracy of illumination measurement in image sensors.
Implementation Method 1
image acquisition cells, called pixels, comprising a photodiode used in reverse
Implementation Method 2
an amplifier transistor having: its gate connected to the first node, a conduction node connected to an output of the cell, and a control node of a back gate voltage, the amplifier transistor being configured so that its threshold voltage varies as a function of the back gate voltage
Data Source
Figure 1A~1B
Figure 1C
Figure 2
AI summary
This description relates to an electronic circuit comprising image acquisition cells (210), each cell having: - a photodetector (PD) connected to a first node (SN) of the cell; - an amplifier transistor (SF) having: its gate connected to the first node, a conduction node connected to an output (250) of the cell, and a control node for a back gate voltage, the amplifier transistor (SF) being configured so that its threshold voltage varies according to the back gate voltage of the amplifier transistor (SF); the circuit comprising at least one control circuit configured to adjust a voltage applied to the control node of the back gate voltage of the amplifier transistor (SF) of one of the cells as a function of a voltage (VX) present at the output of the cell and a reference voltage (VREF).