JFET Op-Amp Input Stage With Matched Currents and Local Feedback
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Solution Overview
Problem
JFETs are not commonly used in electronic circuits due to lower gain, difficulty in manufacturing with tight specifications, large threshold voltage dispersion, and challenging gate voltage requirements, despite having advantages like radiation resilience.
Innovation Solution
An all-JFET operational amplifier design with an input stage using matched current sources and local current feedback to stabilize drain currents and voltages, maintaining equality across a range of operating temperatures, thereby reducing amplifier offset and thermal drift.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If JFETs are used in amplifiers, then radiation resilience is improved, but gain is reduced
Solution Approach 1:
The patent implements local current feedback through a feedback transistor connected between the drain and gate of the input JFET. This feedback mechanism stabilizes the operating point and compensates for the inherently lower gain of JFETs, allowing the amplifier to achieve acceptable gain levels while preserving the radiation resilience advantage of JFET technology.
2Reliability
If JFETs are used in amplifiers, then radiation resilience is improved, but manufacturing precision is worsened
Solution Approach 1:
The patent employs parameter changes by using multiple matched transistor pairs (Q1a-Q1b, Q2a-Q2b, Q3a-Q3b) with carefully selected characteristics. By changing and matching multiple parameters simultaneously (threshold voltage, transconductance, etc.), the design compensates for the large threshold voltage dispersion inherent in JFET manufacturing, achieving tight overall specifications despite individual device variations.
Solution Approach 2:
The patent applies local quality by creating matched pairs of JFETs and transistors with specific characteristics for different stages of the amplifier. Each pair is optimized for its specific function (input stage, current source, gain stage), allowing the overall circuit to achieve high precision through local optimization rather than requiring all devices to meet uniform tight specifications.
3Reliability
If JFETs are used in amplifiers, then radiation resilience is improved, but device complexity is worsened
Solution Approach 1:
The patent introduces intermediary components (feedback transistor Q3 connected between drain and gate, multiple current source transistors Q2a-Q2b) that mediate the control of JFET gate voltages. These intermediaries automatically adjust gate voltages to maintain proper operating conditions, eliminating the need for complex external gate voltage control circuits and simplifying the overall device complexity.
4Measurement precision
If drain currents are maintained equal across temperatures, then amplifier offset is reduced, but device complexity is worsened
Solution Approach 1:
The patent implements feedback through matched current source transistors (Q2a-Q2b) that automatically adjust to maintain equal drain currents through the input JFETs across temperature variations. This feedback mechanism reduces amplifier offset and drift while using simple transistor-based circuitry rather than complex active compensation networks.
Data Source
AI summary
An all-JFET operational amplifier provides improved accuracy and lower thermal drift than conventional JFET amplifiers. In some examples, the JFET operation amplifier includes an input stage including input transistors supplied with equal drain currents by matched current sources. In some examples, the input circuit is stabilized by local current feedback.