Amplifier Alphabet Soup: Part I, Basics Of Power Amplifier Classes A, AB, B And C

By Bill Slade, Orban Microwave Products

Introduction

Modern RF and microwave engineers have many more types of amplifiers at their disposal to suit specific system needs than their predecessors ever dreamed of. Some are optimized for linearity (for use in high-fidelity signal amplification), others for efficiency (where faithful reproduction is not so important; e.g. for constant carrier level signals). Still other types compromise between the two. Classes of amplifiers can more or less be lumped into two groups: the classical controlled conduction angle amplifiers and the so-called "switching" amplifiers. The first group consists of the high-fidelity Class-A and AB operating modes, the class-B and the reduced conduction-angle Class-C amplifier. The switchers include Class-D, E, F and inverse F.

Class A, AB, B and C are defined by the length of their conduction state over some portion of the waveform, such that the transistor state lies somewhere between "fully on" and "fully off". The waveforms of the current through the transistor collector and the voltage across the collectoremitter (or drain-source) overlap significantly, causing a significant fraction of the DC input power to be converted to heat instead of useful RF power. This is because the transistor conduction state changes in step with the input waveform without any regard for the voltage or current state of the output circuit. Namely, the transistor "switches at any old time" and suffers heat dissipation as a result. Despite this, these amplifier modes are time-tested, reliably applied topologies that are straight-forward to design and are found in all types of commercial communications equipment.