Reference Current Circuit Architecture with PTAT Dependency Elimination
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Solution Overview
Problem
Current-mode circuits face instability due to temperature and device technology dependencies, necessitating stable current sources across a wide range of temperatures and fabrication process settings.
Innovation Solution
A reference current generation circuit utilizing mirrored FET transistor pairs, load isolation transistors, and biasing transistors with specific width-to-length ratios to eliminate proportional-to-absolute-temperature (PTAT) dependencies, combining CTAT and PTAT components to provide a stable reference current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If current-mode circuits are used to achieve higher speed operation, then the operating speed is improved, but the stability of reference current sources deteriorates due to temperature and device technology dependencies
Solution Approach 1:
The patent applies parameter changes by carefully selecting the width-to-length ratios of transistors M1, M2, and M3 to specific relationships that eliminate PTAT dependencies. The W/L ratios are designed such that the temperature-dependent parameters cancel out, achieving stability while maintaining high-speed operation. Specifically, the ratios are chosen to satisfy the condition where the sum of inverse square roots of W/L ratios of M1 and M2 equals the inverse square root of W/L ratio of M3.
2Reliability
If the width-to-length ratios of transistors are selected to eliminate PTAT dependencies, then temperature stability is improved, but the device complexity increases due to precise ratio matching requirements
Solution Approach 1:
The patent applies local quality by making different parts of the circuit (different transistor pairs) have different width-to-length ratios tailored to their specific functions. Each transistor pair is designed with locally optimized W/L ratios that contribute to the overall temperature compensation mechanism, rather than using uniform ratios throughout the circuit.
Solution Approach 2:
The patent combines different transistor types (NFET and PFET) with complementary temperature dependencies to create a composite current source structure. The NFET-based circuit provides PTAT current while the PFET-based circuit provides CTAT current, and their combination yields a temperature-stable reference current.
3Ease of operation
If mirrored FET transistor pairs are used to provide reference currents, then the current mirror function is achieved, but temperature dependencies and device technology dependencies cause current variation
Solution Approach 1:
The patent introduces an intermediary current mirror stage that transfers the temperature-stable reference current from the NFET branch to the PFET branch. This intermediary structure ensures that the stable current is properly replicated and distributed to various circuit blocks while maintaining temperature independence.
Data Source
AI summary
An apparatus includes a plurality of mirrored transistor pairs configured to provide a first output current, and a second output current that is substantially equal to the first output current. The apparatus also includes a load isolation transistor configured to pass the first output current along to a resistive load and a first and a second biasing transistor configured to bias the load isolation transistor with a load biasing voltage. A gate and drain of the second biasing transistor may be connected to a gate of the load isolation transistor and a drain of the first biasing transistor. Furthermore, a source of the second biasing transistor may be connected to a gate of the first biasing transistor. The width-to-length ratio of the load isolation transistor, the first biasing transistor, and the second biasing transistor are selected to eliminate PTAT dependencies in the first output current.