CMOS Image Sensor Pixel NMOS Charge Amplifier Design
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Solution Overview
Problem
Conventional large area CMOS image sensors face challenges with high power consumption, complex circuitry, and reduced pixel fill-factor, leading to increased costs and degraded signal-to-noise ratio, particularly in medical imaging applications.
Innovation Solution
A CMOS image sensor design where each pixel is coupled to a current source via a column signal line, incorporating an internal charge amplifier in a common source configuration with an NMOS transistor and MOS capacitors, minimizing power consumption and maximizing pixel fill-factor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If integrator-type amplifier circuits are used in each pixel, then charge amplification is achieved, but power consumption increases significantly
Solution Approach 1:
The patent changes the operational parameters of the amplifier circuit by using a common source configuration with periodic resetting instead of continuous integrator operation. This allows charge amplification to occur only when needed (during readout), significantly reducing power consumption while maintaining measurement precision
Solution Approach 2:
The amplifier operates in periodic cycles: during the integration phase, the amplifier is reset and held in a stable state (low power); during the readout phase, the amplifier actively amplifies the accumulated charge. This periodic operation mode reduces average power consumption while achieving the required charge amplification
2Measurement precision
If complex circuitry is used for charge amplification, then amplification functionality is improved, but production yield decreases and cost increases
Solution Approach 1:
The patent segments the pixel circuit into distinct functional blocks: photodiode for charge generation, simple switching transistors for control, and a common source amplifier for readout. This segmentation allows each component to be optimized independently and simplifies the overall manufacturing process while maintaining amplification functionality
Solution Approach 2:
The patent extracts the amplification function from the integration process and implements it separately using a common source amplifier during readout. This separation allows the use of simpler, more manufacturable circuit elements while preserving the essential charge amplification capability
3Measurement precision
If larger amplifier circuits are used in each pixel, then charge amplification capability is improved, but pixel fill-factor decreases
Solution Approach 1:
The patent merges the amplifier control logic with the pixel readout timing signals, allowing the same control infrastructure to serve both functions. This integration reduces the additional area required for amplification while maintaining full amplification capability
Solution Approach 2:
The patent implements a dynamic amplifier that is activated only during the readout phase rather than being continuously active. This dynamic operation allows the use of smaller, more area-efficient amplifier circuits that can be quickly switched on and off, preserving pixel fill-factor while maintaining amplification capability when needed
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 reduces power consumption, enhances analog performance with low noise and high linearity, and optimizes pixel fill-factor, making it suitable for low-cost, high-resolution medical imaging applications.
Implementation Method 1
a photodiode that generates a charge in response to a received image portion
Implementation Method 2
a capacitor coupled between the signal line and the photodiode (i.e., such that the capacitor forms a feedback of the common source amplifier)
Data Source
AI summary
A CMOS image sensor in which each column of pixels is connected to a signal line that is coupled to a current source, and each pixel includes a charge amplifier having a common source configuration arranged such that a charge generated by its photodiode is amplified by the charge amplifier and transmitted to readout circuitry by way of the signal line. In one embodiment the charge amplifier utilizes an NMOS transistor to couple the photodiode charge in an inverted manner to the signal line while converting the charge to a voltage through a capacitor coupled between the signal line and photodiode (i.e., forming a feedback of the NMOS amplifier transistor).