PWM Audio Feedback Amplifier With Dual Paths for LC Filter Compensation
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Solution Overview
Problem
Class D amplifiers face challenges with high power linear amplification due to nonlinearity introduced by demodulation filters, leading to increased Total Harmonic Distortion (THD) and sensitivity to load variations, and existing feedback solutions either fail to reduce distortion or complicate circuit structures with current feedback loops.
Innovation Solution
A new feedback amplifier architecture with at least two voltage feedback paths, each equipped with a respective passive network to compensate frequency poles of the cascade low-pass filter and load, stabilizing the system and reducing distortion without affecting overall loop gain, and incorporating a fixed frequency clock to manage PWM waveforms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a low-pass LC passive filter is used to extract low-frequency spectral content from the PWM modulated signal, then the amplified signal can be obtained, but nonlinearity is introduced that strongly influences the THD and frequency response
Solution Approach 1:
The patent introduces a feedback loop that takes the output signal from the low-pass filter and feeds it back to the input of the PWM modulator. This feedback mechanism compensates for the nonlinearity introduced by the filter by comparing the actual output with the desired input and adjusting the modulation duty cycle accordingly, thereby reducing THD and stabilizing frequency response despite load variations
Solution Approach 2:
The patent introduces an intermediary feedback path that mediates between the filter output and the modulator input. This intermediary loop allows the system to indirectly control and correct the filter's nonlinear effects without requiring direct modification of the filter itself, enabling compensation while maintaining the filter's essential function
2Measurement precision
If the LC filter is connected in series to the load, then filtering is achieved, but the frequency response becomes dependent on the load characteristics
Solution Approach 1:
The feedback loop continuously monitors the actual frequency response through the filter output and adjusts the PWM duty cycle to compensate for load-dependent variations. This creates a closed-loop system that maintains stable frequency response across different load conditions by dynamically correcting deviations caused by load changes
3Manufacturing precision
If feedback is introduced to reduce distortion and control frequency response, then THD reduction is achieved, but the loop gain must be reduced to stabilize the system due to outphasing introduced by the LC pair
Solution Approach 1:
The patent addresses the phase stability issue by operating the feedback loop in the duty cycle domain rather than directly in the voltage domain. This dimensional change in the control variable allows the system to achieve distortion reduction while maintaining higher loop gain, as the duty cycle modulation provides an additional degree of freedom for phase compensation
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
This architecture significantly reduces nonlinearity and THD by two orders of magnitude, allows for independent frequency response from load variations, and minimizes electromagnetic emissions, while using low-cost reactive components and maintaining high loop gain for improved linearity and frequency control.
Implementation Method 1
the information content relative to the amplified signal may then be extracted from the PWM modulated signal by a low-pass LC passive filter
Implementation Method 2
a snubber network SN is connected in parallel to the load such to reduce the load voltage ripple
Implementation Method 3
The core of inductors for audio filtering applications is of a material having a non-negligible hysteresis, therefore the value of the inductance L varies and depends on the current that flows through the winding
Implementation Method 4
A possible modulation appropriate for this objective is the PWM modulation (Pulse Width Modulation)... the modulated signal is a square wave with a fixed frequency and duty-cycle adjusted in function of the signal to be amplified
Data Source
AI summary
A feedback architecture for a PWM switching audio amplifier is, capable of compensating the effects of the demodulation filter through at least two feedback paths of the voltage applied to a load without degrading the overall loop gain of the device. Each of the feedback paths may include a respective network or filter for compensating a respective frequency pole of the cascade low-pass filter+load and establishing a certain band pass. These networks or filters may be passive networks.