Chapter 5 - Basic Bipolar Transistor Amplifiers and Biasing

Abstract

This chapter covers some basic transistor topologies, including the common-emitter amplifier, emitter-follower, common-base, and differential amplifiers. The important issue of biasing-connection of the transistor so that it operates in the forward-active region (FAR) is also discussed in detail. In order to achieve useful amplification from a transistor, the transistor is generally biased in the FAR. This is the region of operation of the transistor where amplification can occur because the transistor provides current gain. Biasing is the process by which the DC operating point of a transistor amplifier is set to a known and repeatable point in this forward-active region. The DC current gain for a transistor varies with temperature and collector current level. The common-emitter amplifier is a widely used gain block. The emitter-follower is a buffer stage with high input impedance, low output impedance, and a gain of approximately unity. The transistor differential amplifier is a ubiquitous building block used commonly as the front-end of an operational amplifier. The differential amplifier is used to amplify the difference between its two inputs, while rejecting the DC value common to the two inputs.