Differential Amplifier Calibration for High CMRR Near-Ground Output
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Differential amplifier circuits face challenges with common mode rejection ratio (CMRR) and output swing limitations due to resistor mismatch and the inability of rail-to-rail amplifiers to truly swing from the supply rail to ground rail, leading to signal clipping and increased costs for precision resistors or trimming.
Innovation Solution
A system utilizing three operational amplifiers with a Darlington transistor and a PNP bipolar junction transistor (BJT) in the feedback path, along with bias resistors, to achieve high CMRR and output swing close to ground without precision resistors, allowing for amplification of differential signals with minimal common mode gain and extended output range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If traditional differential amplifier circuits use rail-to-rail amplifiers, then the amplifier can operate across the full supply voltage range, but the output cannot truly swing from the supply rail to ground rail, causing signal clipping
Solution Approach 1:
The amplifier output stage is segmented into multiple components: a rail-to-rail amplifier for voltage amplification, a PNP BJT for level shifting, and a Darlington transistor for current buffering. This segmentation allows each component to perform its specialized function, enabling the output to swing close to ground without clipping while maintaining full supply voltage operation
Solution Approach 2:
The PNP BJT and Darlington transistor act as intermediary elements between the rail-to-rail amplifier output and the final output node. These intermediaries enable the output voltage to extend beyond the amplifier's direct output range, achieving swing within microvolts of ground while preventing signal clipping through the level-shifting action
2Measurement precision
If precision resistors are used to achieve high CMRR, then the common mode rejection ratio improves, but the cost and manufacturing complexity increase
Solution Approach 1:
The circuit uses standard-value resistors in configurations where their mismatch would normally degrade CMRR. However, the overall circuit topology and transistor matching provide self-correcting effects that maintain high CMRR without requiring precision resistors, eliminating the need for expensive precision components while achieving the desired common mode rejection
Solution Approach 2:
The invention changes the approach to achieving high CMRR by relying on transistor parameter matching (beta matching of Darlington pairs) rather than resistor precision. This parameter substitution allows standard resistors to be used while maintaining high common mode rejection ratio through the dominant transistor matching effects
3Measurement precision
If trimming is performed to achieve high CMRR, then the common mode rejection ratio improves, but the device complexity and calibration time increase
Solution Approach 1:
The circuit achieves high CMRR through self-correcting topological features and transistor matching without requiring external trimming or calibration. The design inherently compensates for component variations, eliminating the need for manual or automated trimming processes and reducing device complexity
4Ease of manufacture
If standard resistors are used instead of precision resistors, then the cost decreases, but the CMRR deteriorates due to resistor mismatch
Solution Approach 1:
The invention shifts the critical matching requirement from resistor parameters to transistor parameters (beta matching). By making the CMRR dependent on transistor characteristics rather than resistor precision, standard-value resistors can be used without degrading the common mode rejection ratio, significantly reducing cost while maintaining performance
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
The solution provides high gain, high CMRR, and the ability to swing the output signal within microvolts of ground, minimizing clipping and achieving high CMRR without the need for precision resistors or trimming, making it suitable for instrumentation signals and analog-to-digital converter applications.
Implementation Method 1
a first and a second amplifier configured for negative feedback
Implementation Method 2
A system utilizing three operational amplifiers with a Darlington transistor and a PNP bipolar junction transistor (BJT) in the feedback path
Data Source
AI summary
The present invention covers novel approaches to the differential amplification of an input signal. Embodiments of the present invention have precise gain, swing to within micro-volts (μV) of ground, and have high CMRR without the need for precision resistors or tuned potentiometers. Embodiments of the present invention are particularly suited for the amplification of an instrumentation signal for delivery to an analog-to-digital converter. Examples of such signals include the product of a strain-gauge front end, a temperature sensor front end, and certain devices for bioelectronics detection. Embodiments of the present invention which are systems for amplification of a differential input signal can comprise a differential input stage transconducing a differential voltage input signal into a single-ended intermediate current signal using a follower transconductance amplifier, and a single-ended output stage comprising an amplifier producing an output voltage across a resistor network that forms a negative feedback network of the amplifier.