Cascaded MOSFET Current Mirrors for High-Accuracy Sensing
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current current sensing and measurement technologies face limitations in accuracy and power dissipation, especially when measuring large currents, due to challenges in manufacturing low resistance values and matching MOSFETs, leading to limited dynamic range and high power dissipation.
Innovation Solution
The use of cascaded MOSFET current mirrors with controlled gain factors in each stage to improve current measurement accuracy by reducing the overall gain, achieving precise matching and minimizing power dissipation through voltage regulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a single-stage MOSFET current mirror with high current gain M is used, then the dynamic range is extended, but the matching accuracy between MOSFETs deteriorates due to layout issues and proximity effects
Solution Approach 1:
The patent divides a single high-gain current mirror stage into multiple lower-gain stages. Each stage has a moderate current gain (e.g., M1=10, M2=100) rather than one extremely high gain stage (M=1000). This segmentation allows each MOSFET pair to be better matched within its stage while achieving the same overall dynamic range through cascaded stages.
Solution Approach 2:
The patent introduces intermediate current mirror stages between the input and output. These intermediate stages act as mediators that break up the direct high-gain relationship, allowing each stage to operate with optimized matching conditions while collectively achieving the required dynamic range.
2Measurement precision
If bipolar transistors are used in a current mirror, then current measurement functionality is achieved, but power dissipation increases due to emitter-base voltage drops
Solution Approach 1:
The patent replaces bipolar transistor current mirrors with MOSFET-based current mirrors. MOSFETs use voltage-controlled current channels rather than the emitter-base junctions of bipolar transistors, eliminating the inherent 0.6-0.7V voltage drop and associated power dissipation while maintaining current mirror functionality.
3Measurement precision
If very low resistance current measurement resistors are used, then parasitic voltage drop is minimized, but manufacturing precision becomes difficult and expensive
Solution Approach 1:
The patent extracts the current measurement function from the voltage-based resistor method and implements it through current-based MOSFET current mirrors. This eliminates the need for precision low-value resistors entirely, as the current mirrors directly sense and replicate currents without requiring precise resistance values.
Data Source
AI summary
A method and apparatus for current sensing and measurement employs two cascaded MOSFET current mirrors, wherein the mirrored current leaving the first current mirror is fed to the input of the second current mirror. Each current mirror contains a high current MOSFET and a low current MOSFET, connected source-to-source and gate-to-gate. The MOSFETs are matched so that drain-to-source current flowing in the high current MOSFET is proportional to the drain-to-source current flowing in the low current MOSFET. The ratio of high current to low current for each current mirror is M, where M is 100 or less. Voltage biasing networks are employed to maintain constant drain-to-source voltages for both MOSFETs in each current mirror.