Class-AB Feedback Circuit Stabilization Near Unity-Gain Pole
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Solution Overview
Problem
Negative feedback in class-AB amplifiers introduces a pole into the frequency response near or within the unity-gain bandwidth, adversely affecting stability, which is inherent due to parasitic capacitance.
Innovation Solution
Incorporating additional components into the negative feedback loop to shift the pole position, such as using a resistor and capacitor combination, and splitting input transistor pairs to mitigate or negate the pole.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If negative feedback is implemented in class-AB amplifiers, then amplifier performance is improved, but stability deteriorates due to pole introduction near unity-gain bandwidth
Solution Approach 1:
A feedback transistor is introduced as an intermediary component in the negative feedback path. The feedback transistor's gate is connected to the output, source to ground, and drain to the feedback network, acting as a mediator to shape the frequency response and mitigate the destabilizing pole effect while preserving the performance benefits of negative feedback
Solution Approach 2:
The invention changes the electrical parameters of the feedback network by adding the feedback transistor and associated resistors. This modifies the feedback factor and frequency-dependent impedance characteristics, shifting the pole position away from the unity-gain bandwidth region and improving stability margins
2Stability of the object's composition
If additional components are added to the feedback loop to shift pole position, then stability is improved, but device complexity increases
Solution Approach 1:
The feedback transistor serves multiple functions simultaneously: it provides frequency compensation to shift the pole position, maintains the negative feedback action for performance improvement, and integrates with existing amplifier stages. This multi-functionality achieves stability improvement without proportionally increasing complexity
3Stability of the object's composition
If input transistor pairs are split to mitigate pole effects, then stability is enhanced by reducing ringing and oscillation, but manufacturing precision requirements increase
Solution Approach 1:
The input transistor pair is segmented into separate transistors with independent biasing and feedback paths. This segmentation allows each transistor to be optimized independently and reduces the coupled pole effects that cause ringing and oscillation, enhancing stability while managing manufacturing tolerances through modular design
Data Source
AI summary
Aspects of the description provide for a circuit. In some examples, the circuit includes a input pair of transistors, a bias transistor having a bias transistor gate, a bias transistor drain, and a bias transistor source, the bias transistor drain coupled to the input pair of transistors and the bias transistor source coupled to ground, and a resistor coupled between the bias transistor gate and the input pair of transistors.