Semiconductor Basics

9Transistor Biasing Circuits

OBJECTIVES OF THIS CHAPTER

I have already explained (almost) all the components that we need to build very complicated electronic devices using semiconductors. As I said before, Forbes has calculated that as of May 2014 there were 2 913 276 327 576 980 000 000 transistors shipped. This is about 3 × 10²¹. Just for comparison the human body has between 30 and 70 trillion cells or at least 3 × 10¹³. There are100 million more transistors in the world that there are cells in the human body.

In this chapter I explain how we use these transistors, how we bias them, so they generate working and useful circuits, and how we stabilize them as the temperature, currents, and applications vary. We use the term “bias” or “biasing” to indicate the process of connecting the transistor to electrical sources and other components so the transistor operates with optimal performance. I concentrate in this chapter on the basic operation of a single transistor, but I will use these concepts in the following chapters to understand how semiconductor‐based devices work. I will also discuss the operational amplifier, a “single component”, which is more stable and easier to insert in a circuit without much need for design and calculations.

9.1 Introduction

One thing I want to remind you of before I discuss the different transistor biasing methods is that the transistor parameters are not constant. The characteristics of transistors change drastically with temperature ...