Class-D Amplifier Feedback Loop for Duty Ratio Distortion Suppression
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Solution Overview
Problem
Digital Class-D audio amplifiers face challenges in achieving low distortion due to power stage non-idealities and power supply noise, which limits their performance and requires expensive components, while traditional feedback methods do not significantly improve digital designs beyond 0.002% THD+N.
Innovation Solution
A novel distortion suppression feedback loop using discrete time signal processing monitors the effective duty ratio at the power stage output and adjusts gate drive timing to counteract non-idealities and noise, achieving low distortion without the need for expensive power stages and supplies, utilizing a combination of digital modulator and power stage feedback loops.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If feedback is used to suppress distortion in digital Class-D amplifiers, then distortion performance improves, but the improvement is limited and does not reach the sub 0.002% THD+N levels achieved in analog designs
Solution Approach 1:
The feedback system is segmented into two independent loops: an outer loop that monitors the filtered output signal for distortion suppression, and an inner loop that monitors the switching node for power stage non-ideality compensation. This segmentation allows each loop to be optimized independently and achieve better overall performance than a single feedback loop
Solution Approach 2:
A duty cycle correction block serves as an intermediary that processes the error signal from the outer feedback loop and generates corrected duty cycle values. This intermediary component translates the distortion error into appropriate corrections for the PWM generator, enabling precise control without requiring complex direct feedback to the power stage
2Manufacturing precision
If expensive power stages and power supplies are used to achieve low distortion in open-loop configuration, then distortion performance improves, but cost increases significantly
Solution Approach 1:
A dual-loop feedback system is implemented where the outer loop monitors the filtered output and generates duty cycle corrections to suppress distortion, while the inner loop monitors the switching node to compensate for power stage non-idealities. This feedback approach enables the use of less expensive power stages and supplies while maintaining low distortion performance
Solution Approach 2:
The system replaces the need for expensive, high-precision analog power stages with a digital PWM-based power stage that uses feedback control to achieve similar or better distortion performance. The digital modulator and feedback control system substitute for the inherent precision that would otherwise require expensive analog components
3Ease of manufacture
If digital signal processing is used in the modulator, then cost is reduced compared to analog designs, but distortion performance does not improve to match analog feedback systems
Solution Approach 1:
The duty cycle correction block acts as an intermediary that bridges the digital modulator and power stage, processing distortion error signals and generating corrected duty cycle values. This intermediary enables the digital modulator to achieve low distortion performance without requiring expensive analog components
Solution Approach 2:
The system dynamically changes the duty cycle parameter based on feedback from the outer loop. The duty cycle correction block adjusts the duty cycle values in response to distortion errors, enabling the digital modulator to compensate for non-linearities and achieve low distortion performance through parameter optimization rather than expensive hardware
Data Source
AI summary
A digital Class-D amplifier distortion suppression circuit design is disclosed. A distortion suppression feedback loop is described to improve audio performance by suppressing output stage non-linearity and improving power supply noise rejection achieving reduced THD+N. The feedback loop is placed around the power stage. It forces tracking between the audio band signals at the input and output of the power stage by automatically adjusting the gating signal timing based on sensed effective duty ratio error. Error sensing and compensation are performed using techniques that lend to simple circuit implementation.