Single-Ended Op-Amp Leakage Compensation for Offset Cancellation
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Solution Overview
Problem
Single-ended operational amplifiers with n-well-poly capacitance for Miller compensation suffer from offset voltage due to parasitic diode leakage current, which is temperature-dependent and difficult to calibrate, affecting their stability and settling behavior.
Innovation Solution
A leakage current compensation circuit is introduced, using a diode element and current mirrors to balance the leakage currents drawn from both output nodes of the amplifier input stage, effectively canceling out the offset voltage by amplifying the secondary current to match the primary leakage current, thus stabilizing the operational amplifier.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If n-well-poly capacitance is used for Miller compensation, then the operational amplifier achieves stability and desired settling behavior, but offset voltage occurs due to parasitic diode leakage current
Solution Approach 1:
The patent introduces a compensation capacitance with an associated parasitic diode that generates a leakage current. This leakage current is intentionally designed to counterbalance the leakage current from the Miller compensation capacitance, thereby compensating for the offset voltage. The harmful effect (leakage current) is converted into a beneficial compensation mechanism that stabilizes the operational amplifier's offset.
2Object-generated harmful factors
If the leakage current is compensated by adapting the design of the op-amp, then the offset voltage can be reduced, but the gain-bandwidth product cannot be changed as it is given by the specification
Solution Approach 1:
The patent separates the offset compensation function from the main operational amplifier design by introducing a distinct compensation capacitance and associated leakage current generation circuit. This segmentation allows the Miller compensation capacitance to maintain its specified gain-bandwidth product while the separate compensation mechanism independently addresses the offset voltage, preserving design flexibility.
3Object-generated harmful factors
If the operational amplifier is calibrated to remove offset voltage, then the offset can be reduced, but calibration is almost impossible due to exponential temperature dependence
Solution Approach 1:
The patent implements a self-compensating mechanism where the compensation capacitance's parasitic diode automatically generates a leakage current that matches and counterbalances the Miller capacitance's leakage current. This self-service approach eliminates the need for external calibration, as the compensation occurs automatically through the inherent temperature-dependent characteristics of the matched parasitic diodes.
4Stability of the object's composition
If a larger Miller capacitance is used to improve stability, then the stability increases, but the leakage current increases proportionally to the capacitance value
Solution Approach 1:
The patent introduces a compensation capacitance with an associated parasitic diode that generates a counterbalancing leakage current. This compensation leakage current acts as an anti-weight to the Miller capacitance's leakage current, neutralizing its harmful effect. The relationship is expressed as Voff = αC/gm, where the compensation mechanism counteracts the αC term proportional to the capacitance value.
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 independently compensates for temperature and production spread variations, enhancing the operational amplifier's stability and resistance to parameter changes, while maintaining rail-to-rail output voltage linearity.
Implementation Method 1
The parasitic diode has a leakage current which is proportional to the diode area which is proportional to the capacitance value
Implementation Method 2
Single-ended two (or more) stage op-amps need Miller compensation for stabilizing the op-amps
Data Source
AI summary
An electronic device includes an operational amplifier, with the operational amplifier having an amplifier input stage coupled with a first output node to an amplifier output stage. A compensation capacitance is connected between an output node of the amplifier output stage and the first output node of the amplifier input stage, thereby operating as a compensator for stabilizing the operational amplifier. The compensation capacitance provides a parasitic diode drawing a first leakage current from the first output node of the amplifier input stage, a leakage current compensation circuit being coupled to the first output node of the amplifier input stage and coupled to a second output node of the amplifier input stage for drawing a first current from the first output node and a second current from the second output node. The leakage current compensation circuit is adapted such that the second current is greater than the first current by an amount corresponding to the first leakage current.