Module 12: Amplifiers

An amplifier is one of the most important building blocks in all of electronics, and it appears in virtually every piece of equipment you will ever own or build. A microphone preamplifier boosts a signal of a few millivolts to a usable level without adding noise. A sensor amplifier turns the tiny output of a thermocouple or strain gauge into something an analog-to-digital converter can measure. An audio power amplifier takes a line-level signal and delivers watts or hundreds of watts to a loudspeaker. Each of these is built from the same handful of transistor circuits, connected and biased in different ways.

This module builds your understanding from the ground up. You will start with amplifier classes — why some amplifiers conduct all the time while others only conduct for part of each signal cycle, and why the choice matters enormously for efficiency and signal quality. You will then work through the three basic transistor configurations, gain and impedance, biasing, distortion and push-pull output stages, and finish with audio power amplifiers, low-noise preamplifiers, and how to keep any amplifier stable.

Module Overview

At the heart of every modern amplifier is a transistor — either a bipolar junction transistor (BJT) or a field-effect transistor (FET). In the Basic Electronic Components module you learned how these devices work. In this module you will learn how to connect them as amplifiers, how to predict their behavior mathematically, and how to make them work reliably.

Why Amplifier Classes Matter

Not all amplifiers work the same way. A Class A amplifier keeps its transistor conducting the entire time, so it draws current from the supply whether a signal is present or not. This gives very low distortion — ideal for preamplifiers and small-signal stages — but wastes a lot of power. Class AB and Class B push-pull stages are far more efficient and are used in most linear audio power amplifiers. Class D switching amplifiers are more efficient still, which is why they dominate portable speakers, car audio and modern home amplifiers. Choosing the right class for each stage is one of the most important decisions in amplifier design.

The Three BJT Configurations

A bipolar junction transistor has three terminals: base, collector and emitter. In a circuit, one of these terminals is shared between the input and output — and which terminal is shared determines everything about how the amplifier behaves. The common emitter configuration is the most frequently used, giving both voltage and current gain. The common base configuration has a low input impedance and excellent high-frequency performance. The common collector (emitter follower) gives no voltage gain but a high input impedance and low output impedance, making it the standard buffer.

Gain, Biasing and Stability

Gain is the ratio of output to input, expressed as a simple factor or in decibels. When amplifier stages are cascaded, you multiply the gains, or simply add them in dB: a three-stage chain with gains of 20 dB, 15 dB and 10 dB delivers 45 dB. Biasing sets the transistor's DC operating point so that the signal can swing in both directions without clipping, and a good bias circuit keeps that point stable as temperature and transistor gain vary.

Distortion, Push-Pull and Power

Even a correctly biased amplifier produces some distortion. The transistor's transfer characteristic is not perfectly straight, and this nonlinearity generates harmonics and intermodulation products. Push-pull output stages cancel even-order distortion and deliver power efficiently. The final lessons apply everything to audio power amplifiers, low-noise preamplifiers for microphones and sensors, and the parasitic oscillations that can make any amplifier misbehave.

Lessons