Class-D Amplifier Biasing for Single-Ended and Differential Audio Output
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Solution Overview
Problem
Class-D amplifiers operating near a power supply rail compromise dynamic range, signal-to-noise ratio (SNR), and signal-to-distortion ratio (SDR) when attempting to minimize DC current through transducers in single-ended configurations.
Innovation Solution
The implementation of a Class-D amplifier configuration with control logic that selectively switches between single-ended and differential operating states, using a first and second differential modulator circuit to control driver circuits, and modifying the common-mode voltage to optimize quiescent points and reduce DC current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the amplifier channels are biased so that the AC signal midpoint (common-mode voltage) is near a selected power supply rail to minimize DC current through the transducer, then energy waste through the transducer is reduced, but the dynamic range, signal-to-noise ratio (SNR), and signal-to-distortion ratio (SDR) are reduced
Solution Approach 1:
The system separates the biasing function from the signal processing function by introducing a dedicated common-mode voltage biasing circuit that operates independently from the differential signal path. This allows the biasing to be optimized for energy efficiency while the differential modulator maintains optimal dynamic range and signal quality.
Solution Approach 2:
A common-mode voltage biasing circuit is introduced as an intermediary component that adjusts the common-mode voltage level without affecting the differential signal. This mediator allows the system to achieve low DC current through the transducer while maintaining the quiescent point of the modulator control loop away from the power rail, preserving dynamic range, SNR, and SDR.
2Loss of energy
If the modulator control loop is operated with a quiescent point near the selected power rail to bias the amplifier channels, then DC current through the transducer is minimized, but the dynamic range, SNR, and SDR are compromised
Solution Approach 1:
The system dynamically adjusts the common-mode voltage bias independently from the differential signal operation. The biasing circuit can adapt the common-mode level to minimize DC current while the differential modulator maintains its quiescent point at an optimal location for maximum dynamic range and signal quality, allowing both functions to operate at peak efficiency simultaneously.
3Use of energy by moving object
If the amplifier operates in single-ended configuration with biasing near power supply rail, then energy efficiency is improved, but operating characteristics (dynamic range, SNR, SDR) are compromised
Solution Approach 1:
The system changes the common-mode voltage parameter independently from the differential signal parameters. By adjusting only the common-mode voltage to near the power supply rail while maintaining the differential signal's quiescent point at an optimal level, the system achieves energy efficiency without compromising dynamic range, SNR, or SDR.
Data Source
AI summary
A class-D amplifier includes a first differential modulator circuit, a first driver circuit including a first high-side switch and a first low-side switch. An input of the first driver circuit may be coupled to a first output of the first differential modulator circuit so that the first differential modulator circuit controls the first driver circuit. The class-D amplifier may also include a second driver circuit including a second high-side switch and a second low-side switch coupling the second and control logic that selects between a single-ended operating state and a differential operating state of the class-D amplifier circuit. The control logic may selectively determine the input of the second driver circuit in conformity with a current operating state of the class-D amplifier circuit so that the first differential modulator circuit controls the second driver circuit when the differential operating state is selected.