Module 9: Amplifiers

An amplifier is one of the most important building blocks in all of electronics, and it appears in virtually every piece of radio equipment you will ever own. The receiver front end uses amplifiers to boost weak signals without adding noise. The transmitter uses power amplifiers to take a milliwatt-level drive signal and build it up to the legal power limit. The audio circuits in your transceiver use amplifiers to drive a loudspeaker loud enough to hear across a noisy shack. Even the oscillator that generates your carrier frequency uses an amplifier to sustain oscillation. Understanding amplifiers thoroughly is essential to understanding radio equipment from the inside out.

This module builds your understanding from the ground up. You will start with the fundamental concept of amplifier classes — why some amplifiers run all the time while others only turn on for part of each signal cycle, and why the choice matters enormously for efficiency and signal quality. You will then study the three basic transistor amplifier configurations (common emitter, common base, and common collector), learning exactly what each one does to voltage, current, impedance, and phase. From there you will learn how to calculate and measure gain, how to set up a transistor to operate at the correct point on its characteristic curves, how distortion arises and why it matters to your signal quality, and how the push-pull circuit eliminates the worst form of distortion in audio and RF stages alike. The module concludes with the specialized circuits you will actually find inside your radio: RF power amplifiers, low-noise preamplifiers, and the neutralization techniques that prevent RF power stages from oscillating themselves into destruction.

Module Overview

The word "amplifier" literally means something that makes bigger. In electronics, an amplifier takes a small signal at its input and produces a larger version of that signal at its output. The energy for that enlargement comes not from the input signal itself but from a separate DC power supply — the amplifier is really a valve or a tap that controls a much larger flow of power using a small controlling signal. This idea is crucial: the amplifier does not create energy, it transfers energy from the supply to the output signal under the control of the input.

At the heart of every modern amplifier is a transistor — either a bipolar junction transistor (BJT) or a field-effect transistor (FET). In Module 3 you learned how these devices work. In this module you will learn how to connect them as amplifiers, how to predict their behaviour mathematically, and how to recognise them inside the schematics of real radio equipment.

Why Amplifier Classes Matter in Ham Radio

Not all amplifiers work the same way. A Class A amplifier runs its transistor in conduction the entire time, which means it is always drawing current from the supply whether a signal is present or not. This produces very low distortion — ideal for a receive preamplifier — but wastes a lot of power as heat. A Class C amplifier only conducts for a short fraction of each RF cycle, which makes it highly efficient — potentially over 80% — but it produces enormous distortion. That sounds terrible, but in RF power stages this is actually fine: a tuned resonant tank circuit at the output filters the distortion away, and what you get is a clean, efficient RF output. Class C is the standard for CW and FM transmitter stages. For SSB you need linearity, so Class AB is the standard choice — it is a compromise that gives good efficiency while keeping distortion low enough for voice work. Understanding these trade-offs will tell you immediately why the final amplifier stage in your HF transceiver runs hot and why the receive preamp is built completely differently.

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 paths — and which terminal is shared determines everything about how the amplifier behaves. The common emitter configuration is the most frequently used; it inverts the signal and gives both voltage gain and current gain, making it ideal for general-purpose amplification. The common base configuration is used at very high frequencies where its wide bandwidth is essential. The common collector configuration, also called the emitter follower, has unity voltage gain but transforms impedance — it presents a high impedance to the source and a low impedance to the load, making it ideal as a buffer between stages. Each of these configurations appears repeatedly in radio equipment schematics, and once you can recognise them you can predict how they will behave before you even pick up a multimeter.

Gain, Biasing, and Stability

Gain is the ratio of output to input, and it can be expressed as a simple multiplication factor or in decibels. When amplifier stages are cascaded — connected in series — you multiply the gains, or simply add the gains in dB. A three-stage chain with gains of 20 dB, 15 dB, and 10 dB delivers 45 dB total gain. The gain of a transistor stage is not fixed by the transistor alone; it depends critically on how the transistor is biased, that is, how its DC operating point is set. An unbiased transistor sitting at the wrong operating point will clip one half of the waveform and produce severe distortion, or it may not amplify at all. Setting the bias correctly — positioning the transistor at the centre of its linear operating region — is one of the most important practical skills in amplifier design. You will learn voltage-divider bias, the most stable and widely used method, and you will see how to calculate the required resistor values.

Distortion, Push-Pull, and RF Power

Even a correctly biased amplifier produces some distortion. The transistor's transfer characteristic is not perfectly straight — at the extremes of its operating range it bends, and this nonlinearity generates harmonic frequencies (multiples of the original) and intermodulation products (sum and difference frequencies when two tones are present). In a receiver, these spurious products can create ghost signals. In a transmitter, they can cause your signal to splatter into adjacent channels, which is both a technical problem and a regulatory one. Push-pull amplifiers, which use two transistors operating on alternate halves of the signal cycle, cancel even-order harmonics and can produce much lower distortion than a single-ended stage for the same power level. RF power amplifiers add the complication of impedance matching — a transistor optimised for gain has an output impedance of only a few ohms, while the antenna system expects 50 ohms, so a matching network is always required. The tank circuit in a Class C RF amplifier simultaneously filters harmonic products and provides impedance transformation.

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