Low Power High Efficiency Integrated Class-D Amplifier Circuits for Mobile Devices

The consumer’s demand for state-of-the-art multimedia devices such as smart phones and tablet computers has forced manufacturers to provide more system features to compete for a larger portion of the market share. The added features increase the power consumption and heat dissipation of integrated circuits, depleting the battery charge faster. Therefore, low-power high-efficiency circuits, such as the class-D audio amplifier, are needed to reduce heat dissipation and extend battery life in mobile devices. This dissertation focuses on new design techniques to create high performance class-D audio amplifiers that have low power consumption and occupy less space. The first part of this dissertation introduces the research motivation and fundamentals of audio amplification. The loudspeaker’s operation and main audio performance metrics are examined to explain the limitations in the amplification process. Moreover, the operating principle and design procedure of the main class-D amplifier architectures are reviewed to provide the performance tradeoffs involved. The second part of this dissertation presents two new circuit designs to improve the audio performance, power consumption, and efficiency of standard class-D audio amplifiers. The first work proposes a feed-forward power-supply noise cancellation technique for single-ended class-D amplifier architectures to improve the power-supply rejection ratio across the entire audio frequency range. The design methodology, implementation, and tradeoffs of the proposed technique are clearly delineated to demonstrate its simplicity and effectiveness. The second work introduces a new class-D output stage design for piezoelectric speakers. The proposed design uses stacked-cascode thick-oxide CMOS transistors at the output stage that makes possible to handle high voltages in a low voltage standard CMOS technology. The design tradeoffs in efficiency, linearity, and electromagnetic interference are discussed. Finally, the open problems in audio amplification for mobile devices are discussed to delineate the possible future work to improve the performance of class-D amplifiers. For all the presented works, proof-of-concept prototypes are fabricated, and the measured results are used to verify the correct operation of the proposed solutions

Supply modulated GaN HEMT power amplifiers - From transistor to system

Power amplifiers (PAs) for mobile communication applications are required to fulfil stringent requirements concerning linearity while keeping a high efficiency over a wide power range and bandwidth. To achieve this, a number of advanced PA topologies have been developed, mostly based on either load modulation, such as Doherty PAs or load modulation balanced PAs, or on supply modulation such as envelope tracking or envelope elimination and restoration. Supply modulation has an advantage over other topologies as the power range of high efficiency can be realised over arbitrary bandwidths, only limited by the bandwidth of the PA. This does, however, come at the cost of a significantly more complicated voltage supply. Instead of a static supply voltage, the PA needs to be provided with one which is rapidly changing, requiring a supply modulator capable of powering the PA while modulating its supply voltage. This thesis investigates a number of challenges in supply modulated power amplifiers, ranging from the transistor itself to circuit design and system level considerations and focusses on power levels up to 10 W and frequencies between 1 GHz and 4 GHz. Transistors, as the centre-piece of a PA, determine how well the PA reacts to a changing supply voltage. In this work, the traits that make GaN HEMTs suitable for supply modulated PAs were investigated, and gain variation with changing supply voltage was established as an important parameter. This gain variation is described in detail and its impacts on PA performance are discussed. By comparing transistors in literature, gain variation has been demonstrated to be a prevalent characteristic in transistors with GaN HEMTs showing a very wide range of gain variation. Using a small-signal model based on measurements, the voltage dependent behaviour of the feedback capacitance CGD is, for the first time, identified as the origin of small-signal gain variation. This is traced down to the gate field plate which is commonly used to combat surface trapping effects in GaN HEMTs. With this in mind, two different ways of changing the transistor geometry to reduce the impact of gain variation and thus optimise the transistor for operation in supply modulated PAs are discussed and demonstrated using a 250 nm GaN HEMT. As a result of the non-linearity of the feedback and gate-source capacitances, the input impedance of GaN HEMTs changes with supply voltage and drive power. This prevents the transistor from being matched at all supply voltages and input powers and introduces phase distortion. Using simulation and measurement, the impact of input impedance on linearity and efficiency of supply modulated power amplifiers is demonstrated on a 2.9 GHz 10 W PA. Careful selection of the input impedance allows improvement of AM/PM distortion of a supply modulated PA with little cost in terms of AM/AM and PAE. I Supply modulators have a significant impact on efficiency and linearity of the ET system. One supply modulator topology with the potential to generate a supply voltage with a high modulation bandwidth is the RF modulator in which the input DC voltage is turned into an RF signal and rectified, resulting in an output voltage which depends on the excitation of the PA. While PAs are well understood in every detail, there are gaps in the understanding of RF rectifiers. Using active load-pull/source-pull measurements, intrinsic gate and drain waveforms of a GaN HEMT operated as a rectifier are demonstrated for the first time. This allows in-detail evaluation of the impact of the gate termination in self-synchronous rectifiers. It also allows detailed analysis of the loss mechanisms in rectifiers and formulation of the required impedances to realise efficient self-synchronous operation, resulting in efficiencies exceeding 90% over wide power ranges. Using waveform engineering, a new type of RF modulator, with potentially very high bandwidths, based on even harmonic generation/injection is proposed. The necessary operating conditions of the rectifier part of the modulator are emulated using an active load-pull/source-pull system to successfully demonstrate that the rectifier behaves as predicted. Using a simple demonstrator, preliminary measurements were conducted and the RF modulator was shown to work, reaching efficiencies up to 78%. As PA and supply modulator are combined, they present impedances to each other. These impedances have a significant impact on the behaviour of both sub-systems. A simple way to characterise both the impedance presented to the PA by the modulator and the impedance presented to the modulator by the PA is described. Using a state-of-the-art modulator, these impedances are measured, the modulator impedance is demonstrated to be close to the simulated value. These measurements also demonstrate that the impedances change significantly with the operating conditions

Topical Workshop on Electronics for Particle Physics

The purpose of the workshop was to present results and original concepts for electronics research and development relevant to particle physics experiments as well as accelerator and beam instrumentation at future facilities; to review the status of electronics for the LHC experiments; to identify and encourage common efforts for the development of electronics; and to promote information exchange and collaboration in the relevant engineering and physics communities

Multimodal Wearable Sensors for Human-Machine Interfaces

Certain areas of the body, such as the hands, eyes and organs of speech production, provide high-bandwidth information channels from the conscious mind to the outside world. The objective of this research was to develop an innovative wearable sensor device that records signals from these areas more conveniently than has previously been possible, so that they can be harnessed for communication. A novel bioelectrical and biomechanical sensing device, the wearable endogenous biosignal sensor (WEBS), was developed and tested in various communication and clinical measurement applications. One ground-breaking feature of the WEBS system is that it digitises biopotentials almost at the point of measurement. Its electrode connects directly to a high-resolution analog-to-digital converter. A second major advance is that, unlike previous active biopotential electrodes, the WEBS electrode connects to a shared data bus, allowing a large or small number of them to work together with relatively few physical interconnections. Another unique feature is its ability to switch dynamically between recording and signal source modes. An accelerometer within the device captures real-time information about its physical movement, not only facilitating the measurement of biomechanical signals of interest, but also allowing motion artefacts in the bioelectrical signal to be detected. Each of these innovative features has potentially far-reaching implications in biopotential measurement, both in clinical recording and in other applications. Weighing under 0.45 g and being remarkably low-cost, the WEBS is ideally suited for integration into disposable electrodes. Several such devices can be combined to form an inexpensive digital body sensor network, with shorter set-up time than conventional equipment, more flexible topology, and fewer physical interconnections. One phase of this study evaluated areas of the body as communication channels. The throat was selected for detailed study since it yields a range of voluntarily controllable signals, including laryngeal vibrations and gross movements associated with vocal tract articulation. A WEBS device recorded these signals and several novel methods of human-to-machine communication were demonstrated. To evaluate the performance of the WEBS system, recordings were validated against a high-end biopotential recording system for a number of biopotential signal types. To demonstrate an application for use by a clinician, the WEBS system was used to record 12‑lead electrocardiogram with augmented mechanical movement information