----- Original Message -----
From: "Rick Karlquist" <richard@karlquist.com>
To: "Jim Lux" <jimlux@earthlink.net>
> Jim Lux said:
>
> > that some ham, who IS an amplifier designer, would take that on.
There's
> > a
> > lot of new amplifier design approaches out there that are efficient,
have
> > better performance, etc., but the ham market (being relatively tiny)
seems
> > to be willing to buy the same old triode, bipolar, and fet designs that
> > have
> > been around for decades.
>
> Besides the DSP predistortion you mentioned, what are these "new amplifier
> design approaches?" Do they work with an arbitrary load impedance? What
> kind of efficiency are you thinking of?
>
Some interesting class E designs have been demonstrated
by Dave Rutledge at Caltech using really cheap MOSFETs.
These are narrowband, however, and also require somekind
of envelope restoration to maintain linearity. Class D seems
to work well at audio frequencies, but I dunno if anyone has
tried extending that technique to the lower HF range. A friend of
mine who used to be a BC engineer mentions that KFI 640 AM
here in Los Angeles gets ~90% efficiency from their solid state
class E 50 KW transmitter. Again, of course, class E doesn't
\lend itself very well to broad bandwidth, so you would probably
need some kind of Alpha 87 like scheme to make a class E
amp work over a broad frequency range. Not sure what Jim
had in mind.
While better efficiency can be critical for portable and mobile
applications, where power availability and total energy use is
important, it generally doesn't matter much for home station
applications (unless you are K1MAN). I would rather see more
focus on linearity. As W8JI points out, the IMD performance
numbers from modern ham gear are embarrassing. With a
move toward a no-code HF license there is going to be ever
increasing pressure on our phone bands. Poor IMD is the last
thing we need, but that is in fact what we are getting. I guess
everyone wants +12V rigs more than they want good IMD. It
sure would be great if someone tackled the problem of
linearizing +12V PA's to a more acceptable level (-45dBc
3rd order for instance).
> BTW, DSP predistortion is not, strictly, an amplifier design technique.
> It has to be applied to the whole transmitter. It won't help with
> an amplifier module intended to amplify the output of an arbitrary
> exciter.
I think I may have been the one who raised DSP predistortion as a
possible design technique, Rick. And yes, your are quite right that
it would need to be applied to the whole transmit chain. I was
envisioning a technique that would sample the amplifier output and
compare this to the DSP modulator output. The DSP would then
adjust the transmit signal to minimize the difference between the
two signals. Although I am not an amplifier designer [I don't count
laying down somebody elses 50 ohm MMICs on a substrate as
being an amplifier designer :):)], so I can't speak to the details, I think
this is the jist of how a number of "linearizers" work. There are other
techniques as well (creating similar distortion products in a parallel
RF chain and then adding them out-of-phase with the primary
signal).
73 de Mike, W4EF...............................
>
>
> Rick N6RK
> (who IS an amplifier designer)
>
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>
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and ask for Sherman, W2FLA.
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