Class AB Amplifier Common-Mode and Quiescent Current Control
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Solution Overview
Problem
The existing class AB amplifier circuits face instability due to coupling between quiescent current control and common mode potential control, leading to impaired performance characteristics, especially when bandwidths of the two control loops are similar.
Innovation Solution
A combined control path is introduced to manage both quiescent currents and common mode potential of the differential output signal, allowing for a consistent and large bandwidth without coupling issues, utilizing replicas of output transistors and a control amplifier with additive signal components representing these parameters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If separate feedback paths are used for quiescent current control and common mode potential control, then both parameters can be controlled independently, but coupling between the control loops causes instability and performance degradation
Solution Approach 1:
The patent combines quiescent current control and common mode potential control into a single unified feedback path. The control amplifier receives both the quiescent current signal and common mode potential signal as inputs and generates a combined control output, eliminating the coupling instability between separate control loops while maintaining control over both parameters through one integrated structure.
2Reliability
If very different bandwidths are used for the two control loops to avoid coupling instability, then stability is improved, but the parameter controlled by the slower loop is controlled less accurately
Solution Approach 1:
By merging both control functions into a single feedback path with one control amplifier, the patent achieves both high stability and high control accuracy. The unified structure allows both quiescent current and common mode potential to be controlled with the same bandwidth, avoiding the accuracy degradation that occurs when one loop is deliberately made slower.
Solution Approach 2:
The control amplifier serves multiple functions simultaneously: it controls both quiescent current and common mode potential through a single unified control mechanism. This multi-functionality allows the system to achieve accurate control of both parameters without requiring separate specialized control loops with different bandwidths.
3Reliability
If a combined control path is used for quiescent current and common mode potential, then bandwidth consistency and stability are improved, but the control stage complexity increases
Solution Approach 1:
The patent reduces overall complexity by merging control functions. Although the control stage handles multiple signals, the unified feedback path eliminates the need for multiple separate control loops and their associated coupling compensation mechanisms, resulting in a net reduction of system complexity despite the multi-functional control amplifier.
Data Source
AI summary
It is an object of the present invention to provide an amplifier circuit that supplies a differential output signal (Voutp−Voutn) with a stable common mode potential (½×(Voutp+Voutn)) and a stable amplification characteristic. An essential feature of the invention is a control path (24, 26, 30, 28, 36, 38, 34, 32) feeding back into a control stage of the amplifier circuit for the combined control of the quiescent currents that flow through the output transistors (T1-T4), and of the common mode potential of the differential output signal. By means of this combination of two control functions in one and the same control path (24, 26, 30, 28, 36, 38, 34, 32) any coupling between separate control loops is avoided.