Operational Amplifier Offset Trimming Without Temperature Drift
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Solution Overview
Problem
Operational amplifiers with metal oxide semiconductor transistor differential input stages face challenges in achieving low offset voltage (VOS) in the microvolt range with a low temperature coefficient, as existing trimming methods can alter the temperature coefficient of VOS, which is undesirable for high-performance applications.
Innovation Solution
The operational amplifier employs a bias current structure for pMOS and nMOS differential pairs, with a constant and temperature-proportional portion, allowing for trimming of VOS without changing its temperature coefficient, using current sources to adjust the bias current through transistors and maintaining transconductance constancy across temperatures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If trimming is applied to reduce offset voltage to microvolt range, then offset voltage is improved, but temperature coefficient of offset voltage changes (which is undesirable)
Solution Approach 1:
The patent changes the temperature dependency parameter of the trimming circuit to match the temperature dependency of the main circuit. By making the trimming circuit temperature-proportional like the main circuit, the temperature coefficient of the offset voltage remains unchanged after trimming, resolving the contradiction between achieving low offset voltage and maintaining stable temperature coefficient.
Solution Approach 2:
The patent introduces a temperature-proportional trimming circuit as an intermediary that mirrors the temperature behavior of the main circuit. This intermediary trimming circuit ensures that the correction applied does not introduce additional temperature coefficient changes, thereby preserving the original temperature stability characteristics.
2Adaptability or versatility
If complementary input pairs are used for rail-to-rail operation, then input voltage range is improved, but offset voltage matching becomes more difficult
Solution Approach 1:
The patent applies a universal temperature-proportional trimming approach that works for both pMOS and nMOS differential pairs. The same temperature-proportional trimming circuit topology is used across different input pairs, providing a unified solution that maintains offset voltage matching accuracy while supporting rail-to-rail input operation.
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
This approach effectively reduces VOS to the microvolt range with an unchanged temperature coefficient, improving performance by maintaining stability across temperature variations and input voltage ranges.
Implementation Method 1
The first bias current includes a first portion (Ict_p) that is constant with temperature and a second portion (Iptat_p) that is proportional to temperature. The ratio of the first and second portions generates a first transconductance (gm) of the first differential pair (pMOS pair) that is substantially constant for temperatures over a range.
Implementation Method 2
The first trimming circuit can be configured (e.g., by adjusting/switching the first current source) to trim the first bias current through one of the pMOS transistors in the first differential pair (of pMOS transistors). The trimming corrects (i.e. adjusts towards zero volts) a first voltage offset (VOS) of the first differential pair.
Data Source
AI summary
An operational amplifier is disclosed. The operational amplifier activates/couples either a first or a second differential pair of transistors to an input based on the input voltage. The first and second pair of transistors are each biased with a current having a first portion that is constant with temperature and a second portion that is proportional to temperature. By adjusting the ratios of the first and second portions, the transconductance of each differential pair may be made relatively constant with temperature. Each differential pair is coupled to a trim current source that is adjusted to reduce the voltage offset at each output. The resulting voltage offset for the operational amplifier is relatively constant over a range of input voltages and has temperature coefficient unaffected by the trimming process.