Maximum power transfer and percent efficiency: Which is more important?

The amount of power transferred from a source to a load is maximized when the load impedance is made equal to the source impedance. This is called “impedance matching.” However, the percent efficiency is not at a maximum when impedances are matched. In fact, when the source impedance is held constant, the efficiency continues to increase as the load impedance increases. These two concepts are frequently confusing to those who study the electronics of NMR instrumentation. The resolution of this apparent dichotomy lies in understanding the voltage drops that exist across the impedances and the power consumed by them and it lies in elucidating the term “percent efficiency”—all of which are discussed in this article. © 1997 John Wiley & Sons, Inc.