RF Power Amplifier Efficiency: Big Challenges for Designers

The basic laws of thermodynamics ensure that no electronic device can achieve 100% efficiency– although switch-mode power supplies come close (approaching 98%). Unfortunately, anything that generates RF power cannot presently boast such near-ideal performance as there are simply too many impediments to converting DC power into RF power, from losses incurred throughout the signal path, to operating frequency, the inherent characteristics of the device, and others. The result, as an article in MIT Technology Review uncharitably put it, is “a grossly inefficient piece of hardware.”

Not surprisingly, every manufacturer of RF power products, from semiconductors to amplifiers to transmitters, along with universities and the Department of Defense, spends enormous amounts of time and money every year to increase the efficiency of RF power generation. And for good reason: Even small increases in efficiency increase operating time in battery-powered products and reduce the annual electricity bills of wireless base stations. Figure 1 shows just how much the RF portion of a base station contributes to power consumption.

2: Add up how much the various RF-related base station components contribute to power consumption and the result is a very big number. Source: Globecom 2010, R. Grant and S. Fletcher.

Fortunately, these efforts are delivering results that continue to increase RF efficiency every year, some at the device level and others through use of techniques such as envelope tracking, digital pre-distortion/crest factor reduction schemes, and higher classes of amplifiers beyond the ubiquitous Class AB.

A major change in amplifier design, which has in 5 years become the standard in base station amplifiers, is the Doherty architecture. Essentially dormant, it has been used in only a few applications since it was invented by W.H. Doherty of Bell Labs (then a part of Western Electric) in 1936. Doherty's research produced an amplifier architecture that delivers very high power-added efficiency with input signals that have high peak-to-average ratios (PARs). In fact, when properly designed, a Doherty amplifier can produce increases in efficiency of 11% to 14% when compared to standard parallel Class AB amplifiers.

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