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CHAPTER ONE

INTRODUCTION

1.1 History of power amplifiers

Vacuum tubes (valves) amplifiers were by far the dominant until the 1960s, when semiconductors (transistors) started taking over for performance and economic reasons, including heat and weight reduction and improved reliability. At first all silicon power transistors were NPN, and for a time most transistor amplifiers relied on input and output transformers for push-pull operation of the power output stage. These transformers were as always heavy, bulky, expensive, and non-linear, and added insult to injury as their LF and HF phase-shifts severely limited the amount of negative feedback that could be safely applied. Proper complementary power devices appeared in the late 1960s, and full complementary output stages soon proved to give less distortion than their quasi-complementary predecessors as mentioned.

1.1.1 Vacuum tubes versus solid state designs

The following are disadvantages of vacuum tubes over solid state audio amplifiers as mentioned in [2]

1. Vacuum tube amplifiers cost from 3 to 10 time as much as solid-state designs.

2. Vacuum tubes are heavier and bulkier than solid state designs

3. Vacuum tubes are more fragile compared to solid state

4. Vacuum amplifier do not provide any protection against continuous short-circuit conditions at the output

5. Vacuum tubes amplifier waste more energy than solid state designs, due to the necessity of heating the tube filaments.

1.2 Objectives

1.2.1 Main project objective

The main objective of this project was to design a 100W power amplifier with feedbackpair complementary symmetry output power transistors.

1.2.2 Specific project objectives

•  To design three class AB power amplifiers with feedback-pair complimentary symmetry output driven by an active 3-way cross-over network and a pre-amplifier.

•  The total power of the three power amplifiers should some up to a total of 100W each driving a resistive load of 8 ohms.

•  The filters making up the active network should be second order with Butterworth response comprising of Sallen-Key Topology.

1.3 Problem statement

Many power amplifiers are affected by noise and distortion.  A simple class AB power amplifier with feedback-pair complimentary was designed to overcome the mentioned problems.

1.4 Project scope

Class AB power amplifiers with feedback-pair complimentary was designed. The designed circuit was the simulated using Protues software. The results were then compared and found to be similar.

The circuit was then implemented on PCB after which results were taken and analysis done.