Bridge-Tied-Load Class-D Amplifier Common-Mode Shift at Low Amplitude
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Solution Overview
Problem
Class D amplifiers suffer from zero crossing distortion due to dead time periods, leading to unwanted distortion in the output signal, especially at low amplitudes, which results in increased total harmonic distortion and listener fatigue in audio applications.
Innovation Solution
The amplifier circuitry includes a controller that varies the common-mode component of the driving signals based on the amplitude of the input signal, allowing the common-mode component to be adjusted away from the output signal range that experiences zero crossing distortion, thereby reducing distortion in the driving signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If dead time periods are introduced to prevent shoot-through current, then switch safety is improved, but zero crossing distortion increases
Solution Approach 1:
The patent applies preliminary anti-action by introducing a common-mode signal before the zero-crossing point to counteract the harmful effect of dead-time. The common-mode signal is applied in advance (preliminarily) to prevent the distortion from occurring, rather than correcting it after the fact. This anticipatory measure offsets the shoot-through current issue while maintaining signal integrity during the dead-time period.
Solution Approach 2:
The patent changes the parameter of the driving signal by adding a common-mode component that varies with signal amplitude. This parameter change transforms the fixed dead-time distortion problem into a controllable situation where the common-mode signal level is adjusted dynamically. The common-mode signal level is changed based on the instantaneous amplitude of the audio signal to compensate for dead-time effects.
2Reliability
If dead time periods are introduced to prevent shoot-through current, then switch safety is improved, but total harmonic distortion increases
Solution Approach 1:
The common-mode signal acts as a preliminary countermeasure against the harmonic distortion caused by dead-time. By applying this signal before the zero-crossing event, the system preemptively compensates for the distortion that would otherwise be generated during the dead-time period, thereby reducing total harmonic distortion while maintaining switch safety.
Solution Approach 2:
The dynamic adjustment of the common-mode signal parameter based on input signal amplitude allows the system to optimize performance across different operating conditions. This parameter change strategy reduces harmonic distortion by adapting the common-mode level to match the instantaneous signal characteristics, thereby minimizing the impact of dead-time on total harmonic distortion.
3Reliability
If dead time periods are introduced to prevent shoot-through current, then switch safety is improved, but listener fatigue increases
Solution Approach 1:
The common-mode signal serves as a preliminary corrective measure that prevents the generation of distortion products known to cause listener fatigue. By counteracting the dead-time effect before it manifests in the output, the system eliminates the unpleasant listening experience associated with zero-crossing distortion while preserving the necessary dead-time for switch safety.
4Object-generated harmful factors
If common-mode component is varied to reduce zero crossing distortion, then distortion performance is improved, but device complexity increases
Solution Approach 1:
The controller performs multiple functions: it generates the common-mode signal, monitors the input signal amplitude, and adjusts the common-mode level dynamically. This multi-functionality reduces the need for separate dedicated circuits for each function, thereby limiting the increase in device complexity while achieving distortion reduction.
Solution Approach 2:
The patent changes the parameter (common-mode level) of an existing signal rather than adding entirely new circuitry. This approach leverages the existing output stage and control infrastructure, requiring only modifications to the control signal generation. The parameter change method is more efficient than adding separate correction circuits, thus limiting complexity increase.
Data Source
AI summary
This application relates to amplifier circuitry and, in particular, to class-D amplifier circuits. The application describes amplifier circuitry (400) for receiving an input signal (Sin) and generating first and second driving signals (SoutP, SoutN) for driving a bridge-tied-load. The amplifier circuitry includes first and second class-D output stages (403p, 403n) for generating the first and second driving signals based on the input signal. A controller (406) controllably varies a common-mode component of the first and second driving signals based on an indication of amplitude of the first and second driving signals. The controller varies the common-mode component, at lower signal amplitudes, so the common-mode level of the first and second driving signals is moved away from an operating region that leads to distortion.