Doherty Amplifier

[Mike O'Dell]

Frank Gentges wrote:
> I opened my latest copy of RF Design magazine and found an article about 
> a Doherty amplifier for 900 MHz.  This is a good magazine to get and if 
> you can qualify, it is free.
> 
> There were some very fine Doherty transmitters for AM broadcast that 
> generated 50 kW 24/7.  This is a touchy design to get tuned up but once 
> done, it is very good at amplifying high peak to average ratio signals 
> and did not require the high power audio modulator.
> 
> The article can be found at
> 
> http://rfdesign.com/microwave_millimeter_tech/Amplifiers/706RFDF4.pdf
> 
> George Lemaster can add a lot of info and history of their use with AM 
> broadcast.  As I recall, at least RCA used the design and I think one of 
> these was installed at WTOP in Wheaton MD.
> 
> Frank K0BRA
> 
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the Doherty amplifier topology has been a major life-extender
for DMOS power transistors as RF PAs in cellular service.
a huge problem in cellular base station design is, in fact,
power dissipation. the air conditioning required to remove
the heat from the equipment shelters consumes more power
than the radio gear itself and for large sites, the
available shelter space is maxed-out from the A/C requirements.
thermal considerations also drive the physical size of PA modules
so even with advanced antenna sectorization and active beam-forming,
there's just no more room, space or heat, in many of them.

the big problem is that all the major cellular equipment vendors
have promised next-gen hardware which cannot be realized using
DMOS transistors and everyone is painfully aware of it.
Freescale's product roadmap shows high-voltage GaAs power transistors
as the next step beyond DMOS, but nobody knows how to make those
with adequate yeild, not to mention it requires an entirely
new, or at least distinct, fab line since III/V materials poison
a Si facility.

the one technology that's got everyone's attention is GaN - gallium nitride.
the transistors work well and at much higher frequencies as well as
much higher power densities and efficiencies, but there's the little matter
of a substrait to build them on. DARPA has funded a bunch of work to
come up with viable parts since the mil satellite radios on 3.5GHz need them
really badly. DARPA has funded GaN on SiC (Gallium Nitride on Silicon Carbide
substraits) but the big problem is that SiC is harder to make than GaN!

there's a company down in Research Triangle, Nitronix, that is doing
GaN over standard silicon wafers. the founder did his PhD thesis on developing
techniques to match the thermal coefficients of GaN and Silicon so the parts
don't delaminate when they heat up. seems to be working, too. they have
real parts they are delivering. we could probably arrange to get samples
if someone wanted to build an extra-spicy taco with them.

	-mo