The AM Forum

I'm back from a wintertime buried at work, installing another pair of 2 MW VHF tube amplifiers to be completed next month and turned on. Reading the mags I just saw February's IEEE Microwave Magazine ("Lite" format compared to their Transactions on Microwave Theory and Techniques) there are some interesting overviews of the history and status of Doherty architecture and on Outphasing ("Ampliphase") technique. There is a lot of new work being done with these 1935 AM developments, up into microwave frequencies with the latest solid state devices. 

John .... that is the truth ....I have been reading some papers from doctoral candidates concerning this .... the outphasing technique holds considerable promise for high power am .... audio in to a dsp unit driving 2 class C ampliifiers combining together .... high efficiency and relatively simple circuit ...we need to do this

Or class E amplifiers in outphasing. This has been attempted in some reports, and presents some difficulties, as the ideal match to the PAs as they swing in and out of phase (with modulation) is typically at one sweet spot, and they operate off match otherwise. So the overall efficiency suffers or the dynamic range is poor. Fred "Fritz" Raab wrote about a solution in an IEEE paper in 2009, using asymmetric combining. He got 85% overall over a 10 dB amplitude range, at 1.82 MHz. Others have made various reactive loads that are different for each PA.

Another interesting development in outphasing is to use a 4 port combiner, and the 'waster load' port power is then rectified and fed back into the DC bus.

All of this super tweaking for efficiency is academic for hams, since we don't really need to worry about a few % PA efficiency out of a 375 watt carrier, Its the wireless folk that have to figure how to extend battery life for the next gen 5G or whatever it will be.

And do it with low distortion and wide bandwidth - most certainly somethings hams don't worry about!