Abstract

For an excessive period, Class A and Class AB were the only amplifier classes that are compatible with high quality audio systems. This is because valves were the single active device, and Class B amplifiers generated masses of distortion which made them barely acceptable for numerous purposes. Construction of Class AB amplifiers are initiated to overcome zero crossing distortion in Class B amplifiers commonly called as Crossover Distortion. In order to develop an amplifier with the low distortion of the Class A configuration along with the high efficiency output of Class B configuration is to build an amplifier circuit which is a combination of the previous two classes resulting in an advanced type of amplifier called Class AB amplifier. This paper describes the design and the construction of the Class AB audio amplifier. A proposed approach is outlined and the challenges encountered during developing a well-designed circuit in this project are addressed in this report [10]. Though the amplifier is unsuccessfully functioning at the end of the project, masses of knowledge and skills are attained throughout the Semester 1 and Semester 2. For example, basic skills on methods of using the laboratory instruments such as the oscilloscope, multi meter, signal generator,etc, and designing and simulating circuits on multiple simulation programs such as LTSpice, PSpice, etc. Knowledge on reading schematic circuits and constructing a simple circuit on breadboard are also obtained.

Table of Contents

Declaration of Academic Integrity

Abstract

1 Introduction

1.1 Power Amplifiers

1.2 Classification of Amplifiers

1.3 Class AB Amplifier Operation

2 Industrial Relevance

2.1 Market Analysis

2.2 Target Applications

2.2.1 Automotive head and in-trunk units

2.2.2 Automotive active noise cancellation

2.2.3 Home theater system

2.2.4 Sound reinforcement (SR) system

2.2.5 Public address (PA) system

2.2.6 Portable and home docking stations

3 Theoretical Background

3.1 Basic Performance Specifications

3.1.1 Power Output

3.2 Bipolar Junction Transistor (BJT) Fundamentals

3.3 Crossover distortion

3.3.1 Current Gain

3.4 Class AB Amplifier Biasing

3.4.1 Voltage Biasing

3.4.2 Resistor Biasing

3.4.3 Adjustable Biasing

3.4.4 Diode Biasing

4 Design

4.1 Resistor Biasing

4.1.1 Operation Point

4.1.2 Biasing Resistors

4.1.3 Input Power

4.1.4 Output Power

4.1.5 Power Gain

4.1.6 Maximum output power

4.1.7 Power Efficiency

4.2 Class AB amplifier with biasing diodes

4.2.1 Biasing resistors

5 Experimental Method

6 Results and Discussion

7 Conclusions

Reference List

Books

Periodicals and Academic Journals Articles

Online Materials

Appendices

Appendix 1: Project Specification Form

Appendix 2: Gantt Chart

Appendix 3: Project Safety Risk Assessment

Appendix 4: Ethical Approval Questionnaire

List of Figures

Figure 1‑1: Block Diagram of a Power Amplifier…………………….

Figure 1‑2: Efficiency of Amplifier Classes…………………………

Figure 1‑3: Output Waveform of Class AB…………………………

Figure 1‑4: Basic Operation of Class AB Amplifier…………………..

Figure 2‑1: A head unit……………………………………….

Figure 2‑2: In-trunk unit………………………………………

Figure 2‑3: Location of active noise cancellation……………………

Figure 2‑4: Process of active noise cancellation…………………….

Figure 2‑5: A typical home theater system………………………..

Figure 2‑6: SR system commonly used in live performances…………..

Figure 2‑7: PA system equipment……………………………….

Figure 2‑8: Portable and home docking station…………………….

Figure 3‑1: Crossover Distortion in Class B Amplifier…………………

Figure 4‑1: Class AB amplifier with Resistor Biasing…………………

Figure 4‑2:Class AB Q-Point Cutoff………………………………

Figure 4‑3: Class AB amplifier with diode biasing……………………

Figure 4‑4: LTSpice simulation of the output signal………………….

Figure 6‑1: LTSpice Simulation of reference current, Iref……………..

Figure 6‑2: LTSpice simulation of positive and negative side waveform signal

Figure 6‑3: LTSpice simulation of audio input signal against output signal..

List of Tables

Table 1: Classes of Amplifiers…………………………………………

1         Introduction

This report covers the fields of Analogue Electronics and Technological Plasmas. The main purpose of the project is to design and build a power amplifier which is built using high power semiconductors in Class AB topology. The amplifier is incorporated with a speaker as the output. The circuit involves a combination of Class A and Class B power amplifiers with a biasing stage connected at the base of the complementary transistors. The transistors are barely turned on by means of biasing the base of NPN and PNP transistors with enough voltage. There are several biasing techniques available to achieve the most efficient amplifier which is briefly discussed later. Failed attempts and problems encountered during phases of the project are addressed in this report.

1.1            Power Amplifiers

At least a single amplifier is implemented in nearly all electronic devices today regardless of the existence of several classes of amplifiers. Amplifiers are classified by their function which implies whether the device is a voltage amplifier or power amplifier, and its frequency response characteristics. The working description of an amplifier is acknowledged on the condition that the function and frequency response classifications are established. Nevertheless, each type of amplifiers is not described. This report is mainly concentrated on Class AB amplifiers topology. Class AB are largely favored as audio power amplifiers. Power amplifiers are large signal amplifiers. Generally, the output signal power of an amplifier is enhanced when compared to the input signal power. Vast majority of power amplifiers are commonly fitted as the final stage of amplification to drive a loudspeaker load.

Figure 1‑1: Block Diagram of a Power Amplifier

1.2            Classification of Amplifiers

Amplifier classes are denoted by the sum of output signal diverges within the amplifier circuit throughout one cycle of process once presented by sinusoidal input signal. The classification of amplifiers is solely varied base on their linear operation but with far greater efficiency, whilst the rest are a compromise between linearity and efficiency. Amplifiers are primarily grouped into two classes. The first are the classically organised conduction angle amplifiers developing the usual amplifier classes of A, B, AB and C, which are described by the span of their position of conduction over certain ration of the output waveform, such that the output stage transistor operation lies someplace within being saturated and cut off. Digital circuits and pulse width modulation (PWM) are utilised in the second group of amplifiers to continuously shift the input waveform between “fully-ON” and “fully-OFF” which leads the output waveform firmly into the transistors saturation and cut off regions. These latest “switching” amplifiers consist of class D, E, F, G, S, and T.

Class A, B, C, and AB amplifiers are predominantly built as audio amplifiers when compared to the second group of switching amplifiers. Figure 1-1 below shows the four type of amplifiers and its efficiency. Every class of operation has certain usages and features. No class of operation is superior than any other class. The choice of the finest class of operation is decided by means of amplifying circuit depending on the target application. For example, the greatest class of operation for a turntable may not be the greatest class for a radio transmitter.

Figure 1‑2: Efficiency of Amplifier Classes

These amplifiers vary from a near linear output with low efficiency to a non-linear output with a high efficiency. The characteristics of each type of amplifier are summarised in Table 1-1 below;

Table 1‑1: Classifications of Amplifiers