Power-efficient high fidelity analog audio class D amplifiers

Abstract

Class D amplifiers are increasingly prevalent in audio applications due to its high power-efficiency. Conventional power amplifiers like Class A, Class B and Class AB amplifiers have certain limitation in maximum power-efficiency and conventional silicon output stage transistors may not satisfy the requirements of extremely high efficiency and high speed applications. This Final Year Project aims to use GaN power transistors as the output stage to achieve better performance than silicon transistors. The physical properties of GaN transistors enable them to feature low resistance when operating in triode region and fast transition during switching in Class D configuration. Since the research results of GaN transistors are not sufficient at present, the author mainly targets to build up a Class D system which could be used to verify the properties of GaN transistors.

In this project, physical structure and process technology of GaN transistor is studied to show it has low on-state resistance, zero gate charge and low parasitic capacitance. GaN transistors have been successfully implemented in output stage to replace conventional silicon transistors. Besides, an adjustable triangle wave generation circuit is designed for pulse width modulation. The circuit is simple and effective with the capability to control the amplitude and frequency of the triangle wave. A Class D amplifier with bridge-tied load, pulse width modulation and single-feedback is designed to achieve high power-efficiency as well as good linearity. The 4-layer print circuit board with special consideration for high power, high frequency and low noise application is designed and fabricated. Finally, the testing results show that the whole circuit works properly which could be further used to investigate characteristics of GaN transistors in Class D amplifier.