Closed-Loop Audio Amplifier Calibration for Unity-Gain Stability
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Solution Overview
Problem
The control and design of closed-loop class-D audio amplifiers face challenges due to inconsistencies in component specifications, increasing costs, and limited stability range caused by variations in inductor and capacitor values, leading to instability in the feedback loop.
Innovation Solution
Incorporating a small control signal, such as a pilot-tone, into the feedback loop to regulate the unity-gain frequency, with a calibration circuit that adjusts the gain of the derived signal based on comparisons before and after the pilot-tone inclusion, ensuring the unity-gain frequency remains within a target range, thereby maintaining stability and consistency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If component tolerances are tightened to improve consistency, then manufacturing precision improves, but device complexity and cost increase
Solution Approach 1:
The system performs self-calibration by automatically detecting the actual unity-gain frequency through pilot-tone injection and adjusting the loop filter coefficients accordingly, eliminating the need for manual precision component selection and reducing dependency on tight component tolerances
Solution Approach 2:
The system dynamically adjusts the loop filter parameters (coefficients) based on detected component variations, allowing the control characteristics to adapt to actual component values rather than relying on fixed nominal values with tight tolerances
2Manufacturing precision
If component tolerances are tightened to improve consistency, then manufacturing precision improves, but production cost increases
Solution Approach 1:
The automatic calibration system compensates for component variations without requiring precision components, allowing the use of standard tolerance components while maintaining consistent performance, thereby reducing production costs
Solution Approach 2:
The system performs preliminary calibration during manufacturing or initialization to establish the correct operating parameters based on actual component values, ensuring consistent performance without requiring expensive precision components
3Adaptability or versatility
If external LC filter component values vary, then adaptability improves, but feedback loop stability deteriorates
Solution Approach 1:
The system uses feedback from pilot-tone injection to detect actual loop characteristics and automatically adjusts loop filter coefficients to maintain stability across varying load conditions, ensuring the unity-gain frequency remains within the stable range
Solution Approach 2:
The loop filter coefficients are made dynamically adjustable rather than fixed, allowing the system to adapt its control characteristics in real-time based on detected component variations and maintain optimal stability margins
4Measurement precision
If ideal LC-compensation filter zeros exactly cancel LC low-pass filter poles, then measurement precision improves, but device complexity increases due to component tolerance requirements
Solution Approach 1:
The system automatically detects the actual pole locations through pilot-tone injection and adjusts the compensation filter zeros to match, performing self-calibration that achieves precise pole-zero cancellation without requiring manual precision adjustment
Solution Approach 2:
The system performs preliminary characterization of the LC filter poles through pilot-tone analysis before finalizing the compensation filter parameters, ensuring accurate pole-zero cancellation is achieved based on actual component values rather than nominal values
Data Source
AI summary
Aspects are directed to an amplifier circuit including a signal processing circuit and a calibration circuit. In certain specific embodiments, the signal processing circuit includes a signal combiner and a closed-loop feedback path, and the signal processing circuit is designed to provide a loop transfer function for a derived signal partly representing contributions from an audio input signal, a control or pilot signal having a target frequency range, and a calibration signal. The signal combiner is designed to combine aspects of the control or pilot signal and aspects of the audio input signal, and the calibration circuit is designed to adjust an effective gain of the derived signal in response to whether a unity-gain frequency of a signal in the closed-loop feedback path, as provided via the loop transfer function, is higher or lower than the target frequency range. Consistent therewith and in yet more specific embodiments, such an amplifier circuit can define the target frequency range relative to the transfer function and an associated unity-gain frequency.