High-Performance Class-D Audio Amplifiers

Abstract

This thesis describes the development of high-performance Class-D audio amplifiers, outlining their significance in modern audio systems. The primary aim is to reduce the system cost and size associated around Class-D amplifiers by minimizing the use of off-chip components, while ensuring high performance and audio fidelity.

Chapter 1 introduces audio amplifiers as integrated circuits, highlighting their role in amplifying electrical signals to drive loudspeakers in various applications. It outlines the key factors influencing amplifier design, including system cost and size, output power, efficiency, electromagnetic interference (EMI), and audio fidelity. The chapter briefly discusses two major classes of amplifiers- Class-AB and Class-D amplifiers (CDAs), particularly emphasizing the latter for high efficiency benefits on account of the switching output stage. Additionally, it introduces the two types of speakers that the amplifiers in this work are optimized to drive, the conventional electrodynamic speaker and the increasingly popular piezoelectric speaker. The discussion includes the advantages and disadvantages of each speaker type and how their electrical impedances impact amplifier design.

Chapter 2 delves into the architectural and circuit techniques used in modern CDAs, comparing different output stage topologies and modulation schemes, and their impact on high-frequency PWM energy and ripple current — key measures of the amplifier’s EMI performance. It introduces conventional AD / BD PWM modulation schemes, highlighting their high frequency PWM characteristics from DM and CM perspectives, before exploring more complicated multi-level and multi-phase architectures that aim to reduce the ripple content and EMI. The trade-offs between these modulation schemes in terms of EMI performance and component requirements are examined. It discusses the benefits of increasing the PWM switching frequency above the AM band ( 1.7MHz), which then allows for smaller and cheaper LC filters. Pulse-density modulation (PDM) using a 1-bit delta-sigma modulator (ΔΣM) is covered, emphasizing its benefits in terms of linearity and challenges related to wideband quantization noise and EMI. Lastly, the chapter also addresses the adaptability of CDAs for driving various speaker loads, particularly newer piezoelectric speakers, and discusses innovative techniques for damping LC resonance without external resistors, significantly reducing power consumption and system cost.

Chapter 3 outlines the development of a 28WCDA for automotive applications, employing a hybrid multibit ΔΣM-PWM scheme to achieve high linearity and low EMI in the AM band. The design features a fully-differential 3rd order loop filter, a multilevel non-uniform quantizer, and an H-bridge output stage that operates at a switching frequency above the AM band. This hybrid modulation technique addresses the limitations of 1-bit delta-sigma modulation, and significantly reducing out-of-band emissions while maintaining high linearity across a broad output power range thanks to the high-gain loop filter. The digital and analog circuits in this design, including the loop filter integrators and quantizer, are designed with low-voltage devices to ensure area and power efficiency. In contrast, the high-power output stage and driving circuits are built with more robust high-voltage devices. This prototype amplifier meets the stringent CISPR-25 EMI standards within the AM band using a relaxed LC filter, while also achieving high linearity, dynamic range, and supply rejection. Chapter 4 introduces a CDA that incorporates a dual voltage/current feedback (VFB/ CFB) topology, specifically designed to drive capacitive piezoelectric speaker loads without the need for external damping resistors. This dual-loop structure effectively mimics a series resistor in an LCR network, allowing for resistor-less LC resonance damping, thereby reducing cost, size, and power consumption. Load current sensing, used in the CFB path, is implemented using purely low-side on-chip sense resistors, thereby avoiding high-frequency switching issues and simplifying the readout network. This low-side sensing is feasible due to the push-pull modulation scheme, which is advantageous for a low-power design. Techniques such as CFB filtering and chopping are employed to reduce non-idealities like noise and non-linearity in the feedback paths. The prototype, fabricated in a 180 nm BCD process, can drive a 4 μF load with a peak current of 4.4 A and achieves an idle power consumption of 122 mW. Measurement results confirm the system’s efficacy in damping LC resonance and maintaining high performance, with significant power savings compared to traditional designs using external resistors.

Chapter 5 builds upon the dual feedback architecture introduced in Chapter 4 by incorporating a quadrature chopping scheme to further enhance the linearity and noise performance of Class-D amplifiers. This technique addresses timing skew issues between low-voltage input choppers and high-voltage output choppers, which can degrade signal linearity and increase noise. The quadrature chopping scheme dynamically matches the timing of the choppers, resulting in significant improvements in large-signal total harmonic distortion (THD) and a reduction in noise foldback into the audio band. The chapter presents measurement results that demonstrate the extension in the linear output of the amplifier to close to 95% the full-scale.

Chapter 6 concludes the thesis by summarizing the key findings and contributions of the research. It highlights the original contributions, including the hybrid PWM-DSM modulation scheme, the dual feedback topology for resistor-less damping of capacitive piezoelectric speaker loads, and the quadrature chopping scheme for further improved linearity and noise performance. The chapter also discusses potential future research directions, such as further optimization of EMI performance through advanced modulation techniques, improved current sensing methods, and enhanced feedback accuracy.