Class-AB Amplifier Current Mirroring for Accurate Load Sensing
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Solution Overview
Problem
Conventional class-AB amplifiers face challenges in accurately sensing small load currents due to the use of sense resistors in series with the half-bridges, leading to underutilization of audio speakers and potential damage from over-displacement and overheating, while also suffering from crossover distortion and high quiescent current.
Innovation Solution
A current sensing circuit that mirrors the load current through a resistive element without using sense resistors in series with the half-bridges, allowing for real-time impedance estimation and optimized audio power delivery, using a digital input class-AB amplifier with a pair of half-bridges, a digital communication interface, a digital core circuit, and analog-to-digital converters to generate a digital signal based on the sensed load current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If sense resistors are used in series with the half-bridges to sense load current, then load current can be sensed, but the audio speaker is underutilized and vulnerable to damage from over-displacement and overheating
Solution Approach 1:
The patent uses an intermediary current mirror circuit to sense the load current indirectly. Instead of placing sense resistors in series with the half-bridges, the invention mirrors the current flowing through the output transistors using current mirror transistors, and senses this mirrored current through a single sense resistor connected to the biasing circuit. This intermediary approach allows accurate current sensing without the drawbacks of direct series sensing, enabling both precise measurement and reliable speaker protection.
2Use of energy by moving object
If conventional class-B amplifier configuration is used, then quiescent current is reduced, but crossover distortion increases
Solution Approach 1:
The patent applies parameter changes by implementing a class-AB biasing configuration instead of class-B. The biasing circuit generates bias currents that keep both output transistors slightly conducting even when no signal is present, changing the operating point from class-B (zero bias) to class-AB (small bias current). This parameter change eliminates the crossover distortion inherent in class-B amplifiers while maintaining low quiescent current consumption through efficient current mirror design.
3Object-generated harmful factors
If class-AB amplifier with push-pull configuration is used, then crossover distortion is reduced, but device complexity increases
Solution Approach 1:
The patent merges multiple functions into a single integrated biasing circuit. The biasing circuit simultaneously performs: (1) generating the class-AB bias currents for the output transistors, (2) providing the reference current for the current mirror, and (3) enabling load current sensing through the sense resistor. This consolidation reduces device complexity compared to conventional class-AB amplifiers that require separate biasing and sensing circuits, while still eliminating crossover distortion through the push-pull class-AB configuration.
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
Enables accurate real-time sensing of load currents, optimizing audio power delivery and speaker protection, reducing crossover distortion, and improving sound quality by estimating impedance and adjusting output power accordingly.
Implementation Method 1
mirroring a current flowing through a first transistor of the first half-bridge to generate a mirrored current
Implementation Method 2
flowing the mirrored current through the resistive element, and sensing the load current based on a voltage of the resistive element
Data Source
AI summary
In an embodiment, a class-AB amplifier includes: an output stage that includes a pair of half-bridges configured to be coupled to a load; and a current sensing circuit coupled to a first half-bridge of the pair of half-bridges. The current sensing circuit includes a resistive element and is configured to sense a load current flowing through the load by: mirroring a current flowing through a first transistor of the first half-bridge to generate a mirrored current, flowing the mirrored current through the resistive element, and sensing the load current based on a voltage of the resistive element.