Multi-Stage Amplifier Compensation Without Miller Capacitors
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Solution Overview
Problem
Amplifiers with multiple gain stages face challenges in maintaining loop stability without using Miller capacitors, which can lead to a decrease in cut-off frequency and non-linearity due to significant slew rate issues.
Innovation Solution
The amplifier circuit structure is modified to include a stage of unity gain with MOS transistors, where the input signal is copied at low impedance and used to force the phase of intermediary nodes, eliminating the need for Miller capacitors and maintaining phase margin for stability, while increasing operating speed and high-frequency response.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If Miller capacitors are used to ensure loop stability, then stability is improved, but cut-off frequency decreases and non-linearity increases
Solution Approach 1:
The patent removes Miller capacitors from the amplifier circuit entirely, extracting the problematic stability-compensation mechanism that was causing frequency degradation and non-linearity. Instead, it uses the inherent properties of the multi-stage amplifier structure with carefully designed load capacitors to achieve stability without the harmful effects of Miller compensation.
Solution Approach 2:
The patent employs feedback mechanisms through the interconnected stages where the output of each stage feeds into the next, creating a natural phase compensation effect. The load capacitors on intermediate nodes work with the transconductance amplifiers to provide frequency-dependent feedback that ensures stability without requiring additional Miller capacitors.
2Reliability
If Miller capacitors are used to ensure loop stability, then stability is improved, but non-linearity increases due to slew rate issues
Solution Approach 1:
The patent extracts and removes the Miller capacitors that were generating non-linear behavior through their interaction with the amplifier's slew rate limitations. By eliminating these capacitors, the circuit avoids the charge redistribution effects that cause non-linearity during large-signal transitions.
Solution Approach 2:
The patent changes the compensation approach from using large Miller capacitors to using smaller load capacitors on intermediate nodes. This parameter change reduces the time constants involved in charge redistribution, thereby reducing non-linear effects while maintaining stability through the multi-stage feedback structure.
3Reliability
If additional Miller capacitors are added for three-stage amplifier stability, then stability is improved, but device complexity increases
Solution Approach 1:
The patent makes the load capacitors on intermediate nodes serve dual functions: they provide frequency compensation for stability while also acting as the primary output coupling elements for each stage. This multi-functionality eliminates the need for separate Miller capacitors, reducing overall device complexity.
Solution Approach 2:
The patent merges the stability compensation function with the existing load capacitors of each stage. Instead of adding separate Miller capacitors, the design combines the compensation role with the natural output capacitors, thereby achieving stability without increasing component count or circuit complexity.
Data Source
AI summary
An embodiment of an amplifier circuit comprising a succession of amplification stages having at least a first amplification stage receiving a first signal and a second amplification stage downstream of the first amplification stage; a stage of unity gain capable of receiving the first signal and of providing a second signal corresponding to the low-impedance copy of the first signal; and a third amplification stage having its input connected to the output of the stage of unity gain by a capacitor and having its output connected to the output of the second amplification stage.