Class-D Amplifier PWM Feedback for Distortion Correction
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Solution Overview
Problem
Class-D amplifiers suffer from distortion in their output signal due to non-idealities such as delays, finite rise and fall times, and other imperfections, leading to imperfect translation of the duty cycle input to the voltage on the load, which can result in audible distortion even at low levels like 0.1%.
Innovation Solution
A method and apparatus that includes a reference section generating a low-power reference signal to compare with the main power output, creating an error signal based on the difference, and using this error signal to adjust the PWM signal through a feedback loop to minimize the difference between the power output and the reference signal, thereby reducing distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a class-D amplifier uses switching elements to achieve high efficiency, then power dissipation is reduced, but distortion in the output signal increases due to non-idealities such as delays and finite rise/fall times
Solution Approach 1:
The patent implements a feedback mechanism where the output signal is fed back through a reference section to generate a reference signal. This reference signal is compared with the original input signal to generate an error signal, which is then used to adjust the PWM duty cycle. This closed-loop feedback system compensates for distortion caused by switching non-idealities while maintaining the high efficiency of class-D operation.
Solution Approach 2:
The patent introduces a reference section as an intermediary element that creates a reference signal from the output signal. This reference section acts as a mediator between the output and the control input, allowing the system to compare actual output with desired output and generate correction signals without directly modifying the power switching elements.
2Measurement precision
If the amplifier spends equal time at each level to achieve 0V output, then the average output voltage is correct, but distortion occurs due to non-linearities in the switching transitions
Solution Approach 1:
The feedback loop continuously monitors the output signal and adjusts the PWM duty cycle to compensate for distortion. Even when the amplifier operates at 50% duty cycle to achieve 0V output, the feedback mechanism detects any distortion introduced by switching non-linearities and generates corrective error signals to maintain signal fidelity.
Solution Approach 2:
The correction signal is generated in advance by the reference section and added to the input signal before PWM generation. This preliminary correction adjusts the duty cycle to anticipate and compensate for expected distortion from switching transitions, ensuring more accurate output voltage and better signal fidelity.
Data Source
AI summary
The present application describes an apparatus and method for reducing distortion in a class-D amplifier. The power output section of the amplifier is driven by an adjusted PWM signal, rather than by a PWM signal created directly from the input analog signal. A reference output, designed to closely track the input analog signal, is compared to the amplifier output. The resulting difference is an error signal which is inverted and summed with a second analog signal corresponding to the directly created PWM signal and changes the timing of the voltage transitions of the second analog signal. The changed voltage transitions are used to create the adjusted PWM signal. The inversion of the error signal causes negative feedback which results in the adjustment of the PWM signal being in a direction which reduces the error signal and thus the distortion of the amplifier.