Outphasing Improves RF Amp Efficiency

An MIT spinout company, Eta Devices has come up with a way to use a technology described in a 2009 paper Asymmetric Multilevel Outphasing Architecture for Multi-standard Transmitters.

The MIT Technology Review article Efficiency Breakthrough Promises Smartphones that Use Half the Power says it is currently a lab-bench technology, “but if it proves itself in commercialization, which is expected to start in 2013--first targeting LTE base stations--the technology could slash base station energy use by half.”

The article notes that an integrated circuit version of the technology, which is reported to be still under development, may be able to double smartphone battery.

The technology sounds very similar to envelope tracking, a technology Thomson is using in the Futhura Plus transmitter, which was demonstrated at the 2012 NAB Show. However, instead of continuously varying the supply voltage, this technology varies it in discrete steps and uses outphasing for fine envelope control. Such technology allows the amplifier to reduce power consumption when it isn't running at full output.

“It means you are pulling a lot of energy just to keep the thing on,” said Joel Dawson, Eta Devices co-founder. “And the more data you need to send, the worse it gets. With high data rate communication, you wind up needing far more standby power than signal power. This is why the phone is warm.”

The abstract of the 2009 paper provides a more detailed technical description:

“A 1.95-GHz asymmetric multilevel outphasing (AMO) transmitter with class-E GaN power amplifiers (Pas) and discrete supply modulators is presented. AMO transmitters achieve improved efficiency over envelope tracking transmitters by replacing the continuous supply modulator with a discrete supply modulator implemented with a fast digital switching network. Outphasing modulation is used to provide the required fine output envelope control.

 “A four-level supply modulator is implemented that allows for fast and efficient discrete envelope modulation with up to 28-V supply voltages using low-voltage gate drivers and time-alignment logic. With two class-E GaN PAs that achieve 62.5 percent power-added efficiency (PAE) at 40-dBm peak output power, the AMO transmitter delivers 42.6- dBm peak output power at 1.95-GHz.

“For a 16-QAM signal at 36-dBm output power, the transmitter achieves 44.2/42.8/41.4 percent average system efficiency and 2.0/2.1/3.1 percent EVM for 10/20/40-MHz channel bandwidth, respectively.”

I don't see any reason why this new technology couldn’t be applied to DTV transmitters, but with NXP's announcement of full-UHF band Doherty amplifier (see this week's article “NXP Develops Broadband Doherty UHF Amp”), one wonders if the extra efficiency will be worth the complexity for higher power broadcast transmitters.

Doug Lung is one of America's foremost authorities on broadcast RF technology. He has been with NBC since 1985 and is currently vice president of broadcast technology for NBC/Telemundo stations.