Amplifier Alphabet Soup: Part II, Basics Of Power Amplifier Classes D, E, F And Inverse-F

By Bill Slade, Orban Microwave Products

Introduction
In the last twenty years or so, the increased availability of inexpensive high speed power transistors developed for use in switching power supplies has prompted a revolution in power amplifier design. Voltage regulating switching power supplies avoid the losses encountered in traditional linear "series pass" or shunt regulators by adjusting the on-off transitions of the power transistors; i.e. using the transistors in either a "fully-on" or "fully-off" state to reduce resistive loss. It turns out that these same techniques can yield very efficient power amplifiers as well.

Part I of this series summarized the operation of the classical limited conduction angle amplifier topologies (Class-A, AB, B, C). A common feature of the classical amplifiers is the fact that the transistor spends significant time in its resistive "transition" region as the transistor action tracks the input waveform. This is a source of loss and inefficiency. The "switchers", on the other hand, are designed to swing quickly between a high resistance state to a low resistance state and back again. In this way, little time is spent in the resistive region, thereby maximizing efficiency. The switchers are given the class designators D, E, F and inverse F. Note that switching amplifiers are by definition non-linear with respect to the input signal; that is, amplitude information is not preserved. Various linearization techniques are possible, but that is the possible subject of another article.

As in Part I, we shall focus on a qualitative review of these amplifier class definitions as well as giving some explanation on where inefficiencies appear in each. Some of the advantages and disadvantages of each topology will be presented and the trade-offs will be briefly elaborated in an intuitive manner. The active devices will be assumed to be ideal (i.e. no or only resistive parasitics, no switching delays, etc.) to simplify the exposition. Naturally, the analysis and design of real-world amplifiers must consider the effects of parasitics and imperfections, which are often the major impediment to developing real-world versions of these amplifiers. However, for clarity of exposition, effects of device parasitics and imperfections are ignored in this article.