Introduction

Nearly every electronic device — a radio, a mobile phone, a public-address system, a music system — depends on making a weak electrical signal stronger. A microphone or an antenna produces a signal of only a few microvolts or millivolts, far too small to drive a loudspeaker on its own. The circuit that builds this weak signal up into a usable one is an amplifier, and when it is built around one or more transistors it is a transistor amplifier.

This chapter of Karnataka 2nd PUC Electronics studies how transistor amplifiers work and how they are described and classified. We begin by defining amplification and the idea of faithful amplification — enlarging a signal without changing its shape. We then look at the general parameters that let us compare amplifiers and choose one for a job: current gain, voltage gain, power gain, input impedance, output impedance and bandwidth, including how gain is measured in decibels. Finally we study the three single-stage BJT configurations — common base (CB), common collector (CC) and common emitter (CE) — together with the common-source JFET amplifier, comparing their gains, impedances and phase behaviour.

These ideas are the foundation for the power amplifiers and multistage amplifiers that follow later in the chapter, and transistor amplifiers are a regularly tested topic in the II PUC board exam.