Differential Amplifier Pixel Circuit Threshold Voltage Optimization
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Solution Overview
Problem
The challenge is to reduce the pitch of pixel circuits in solid-state imaging devices while maintaining or increasing the output range of differential amplifiers, as existing solutions either increase circuit scale or are limited by low power supply voltages in fine CMOS processes.
Innovation Solution
A differential amplifier configuration is implemented with an input differential pair, current mirror pair, and constant current source using MOS transistors, where the threshold voltage of the input transistors is higher than that of the constant current source, allowing for reduced pitch without increasing circuit scale and maintaining output range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If the circuit configuration is modified to provide an output buffer after the differential amplifier to increase the output range, then the output range is improved, but the circuit scale becomes large
Solution Approach 1:
The patent changes the threshold voltage parameter of the MOS transistors to extend the output range. Specifically, it sets the threshold voltage of the input differential pair transistors (first and second MOS transistors) to be higher than that of the constant current source transistor (fifth MOS transistor), which allows the output voltage to swing over a wider range without adding buffer circuits.
2Length of moving object
If a fine CMOS process is used to reduce the pitch of the pixel circuit, then the pitch is reduced, but the power supply voltage is low which reduces the output range
Solution Approach 1:
The patent utilizes the parameter change of threshold voltage to overcome the limitation of low power supply voltage in fine CMOS processes. By configuring the input differential pair transistors with higher threshold voltages than the constant current source transistor, the output range is extended despite the low supply voltage, enabling both small pitch and adequate output range to be achieved simultaneously.
Data Source
AI summary
A pixel circuit includes a differential amplifier. The differential amplifier includes a non-inverting input terminal, an inverting input terminal, and an output terminal. The differential amplifier includes an input differential pair including first and second NMOS transistors, a current mirror pair including PMOS transistors, and a constant current source including a fifth NMOS transistor. A threshold voltage of each of the first and second NMOS transistors is higher than a threshold voltage of the fifth NMOS transistor. Further, the threshold voltage of each of the first and second NMOS transistors is higher than a threshold voltage of another NMOS transistor.