# Building Block I – The Amplifier

### Introduction

Many would consider starting with antennas or filters as a better introduction to radio circuit theory.  These are conceptually more simple (although if you dig far enough, antenna theory can be very intimidating as well).  However, I think starting your adventure into the realm of radio with amplifiers to be a superior choice.  Let me explain my reasoning.

Firstly, it forces the novice learner to become familiar with active semiconductors: the transistor in all its formats, including Bipolar Junction Transistors and Field Effect Transistors.  Having an intuitive feel and understanding of these devices if one of the most important skills you need to learn if you want to progress in the technical side of this hobby.  It is a prerequisite to be able to design radio circuits, so might as well get acquainted with them as quickly as possible.

Second, the field of amplifier design theory is incredibly deep and encompasses all aspects of Electronic Engineering and Control Theory.  However, we can start our foray into amplifiers with easy concepts, and then gradually dive deeper and deeper.  You can start this chapter with a high school level of mathematics and science.  But if you stick to it until the end, you’ll have the knowledge of a working engineer.  By the end of this chapter, you will have all the tools at your disposal for RF design, and the rest of the chapters will be a breeze.

This block may seem intimidating, but this is by far the longest and “hardest” one.  Finish this one, and I guarantee the other blocks will be just a formality.  And remember, I’ll be holding your hand the entire way, doing my best to make the steep learning curve as smooth as possible.

### The Amplifier’s Role

Unsurprisingly, an amplifier…amplifies.  An input signal is applied to its entry port, and, ideally, the same signal is found at the output port, albeit at a higher power level.  The amount by which the input signal is amplified is called the amplifier’s gain.  It is expressed in dB (decibels).  We can measure 3 kinds of gain for our amplifier:

• A voltage gain
• A current gain
• A power gain

What we’re most interested in, as hams, is the power gain.

### Where do we Find Them?

Inside a radio circuit, amplifiers can be found:

• before a mixer, to reach its minimum drive level, or to amplify really low level signals
• before the antenna, to transmit more power
• before a speaker, to reach minimum drive level
• after a microphone
• after a mixer
• anywhere you need to raise the power level of a signal

### What Will you Learn?

The following lessons will be broadly divided into the following 5 topics:

1. Active semiconductors and how they work
2. The basic amplifier configurations
3. Open loop amplifiers and their problems
4. Feedback amplifiers (this is a big one)
5. Amplifier stability and S-parameters

Let’s begin!  Next chapter, we’ll learn about the device that has changed the world: the BJT!