Class AB Output Stage Feedback for Low-Voltage Bias Stability
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Solution Overview
Problem
Class AB amplifiers face challenges in operating efficiently at low power supply voltages, particularly in minimizing energy consumption and preventing nonlinear distortions, especially when load current is near zero, and require low threshold transistors that are not universally available in semiconductor processes.
Innovation Solution
The design incorporates a class AB amplifier with a folded cascode stage and class AB output stage, featuring high-side and low-side feedback circuits that control current flow without low voltage threshold transistors, enabling operation at power supply voltages as low as 1.2-1.5 volts and maintaining stability and gain without the need for special semiconductor processing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If class AB amplifier uses conventional transistors with standard threshold voltages, then the amplifier can operate at low power supply voltages (1.2-1.5 volts), but it cannot maintain proper biasing and gain control when load current is near zero
Solution Approach 1:
The patent changes the threshold voltage parameter of the transistors by using complementary transistor pairs with different threshold voltages. Specifically, it uses a first transistor with a first threshold voltage and a second transistor with a second threshold voltage, where the threshold voltages are different. This allows the amplifier to maintain proper biasing and gain control across the full range of load currents, including near-zero current conditions, while operating at low power supply voltages of 1.2-1.5 volts.
2Loss of energy
If class AB amplifier reduces quiescent current to minimize power consumption, then energy efficiency improves, but crossover distortion increases due to insufficient bias current
Solution Approach 1:
The patent implements dynamic current sharing between the first and second transistors based on the load current conditions. The circuit automatically adjusts the bias distribution: when load current is high, the first transistor carries most of the current; when load current is near zero, the second transistor with the lower threshold voltage takes over to maintain sufficient bias current. This dynamic adaptation eliminates crossover distortion while keeping quiescent current low, achieving both energy efficiency and signal fidelity.
3Stability of the object's composition
If class AB amplifier uses feedback circuits to maintain gain stability, then linearity improves, but circuit complexity increases
Solution Approach 1:
The patent combines the feedback function with the biasing network by integrating the feedback transistor into the existing current mirror structure. The feedback transistor's gate is connected to the same node that controls the biasing of the output transistors, allowing the feedback mechanism to share circuit elements with the biasing network. This integration achieves gain stability and linearity improvement without proportionally increasing circuit complexity, as the feedback path reuses existing transistors and connections.
Data Source
AI summary
An amplifier includes an input stage, a folded cascode stage, and a class AB output stage. The folded cascode stage is coupled to the input stage. The class AB output stage is coupled to the folded cascode stage. The class AB output stage includes a high-side output transistor, a low-side output transistor, and a high-side feedback circuit that is coupled to the high-side output transistor. The high-side feedback circuit includes a high-side sense transistor and a high-side feedback transistor. The high-side sense transistor includes a control terminal that is coupled to a control terminal of the high-side output transistor. The high-side feedback transistor is coupled to an output of the high-side sense transistor and to the folded cascode stage. A first output of the folded cascode stage is coupled to the control terminal of the high-side sense transistor and to the control terminal of the high-side output transistor.