Amplifier Alphabet Soup: Part II, Basics of Power Amplifier Classes D, E, F and Inverse-F

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. 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, causing loss and efficiency. The “switchers”, on the other hand, are designed to swing quickly between a high resistance state to a low resistance state and back again. The switchers are given the class designators D, E, F and inverse F.

This Part-II article focuses 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.

Click here to read Part-I.