Dick,
I'm still waiting for a decent SS Amp that runs 1.5 KW out and does
not cost a fortune.
Why wait? Why not build it yourself? The problem is, precisely, that
everyone is waiting for other people to build it and sell it cheaply!
Jump in, develop it yourself, then you can sell it cheaply to lots of
other hams, and get rich from it, even if you earn just a few bucks on
each! You could put the whole competition out of business, if you really
transfer your low production cost to the customers!
Now irony aside, you can REALLY build one yourself, at least a single
one for your own use. Estimated parts cost is maybe 500 dollars,
including the power supply.
I have my own design pretty much finished, the individual modules
tested, but still have to bring all parts together.
Peter,
I am still waiting to see a review of a SS amp that is as good as the
last generation of tube amps when one looks at the generation of
high order IM products.
Do you want such an amp for yourself? I would rather like THE OTHER
hams, specially those that live close to me, to use such amps! ;-)
But the obvious question arises: What's the point of an ultra low IMD
amp, if we will drive it with existing transceivers??? It will just
amplify the high IMD levels of the transceiver! My own Kenwood TS-450
measures an IMD level of -18dB on 10 meters at 90 watts. That's due to
its final stage being grossly misdesigned, rather than to any intrinsic
shortcomings of power transistors. Nobody will ever notice on the air
whether my amp has -28dB or -60dB IMD, when driven by that radio!
And I'm not bashing Kenwood - the other brands don't seem to do better.
Many radios use misdesigned circuits, and try to linearize them by heavy
feedback. Feedback does help a lot, but cannot completely fix a wrong
base design!
With pre-distortion and a totally digital tx, it can be done. With a
linear amplifier using Cartesian or Polar feedback, it can be done,
but there aren't any around of either type.
Those techniques may become useful once we have clean exciters. Maybe
full SDRs will achieve that some day. Until then, a correctly designed
but plain MOSFET amp, using moderate negative feedback, is better than
the exciters.
Take the REAL transfer characteristic of the device and calculate the
higher IM products....
Yes, if you take the strongly bent curve of a MOSFET, without any
linearisation, the result is horrible - just the same as when you do
that exercise with a triode! Both the MOSFET and the triode need to be
used with strong negative feedback, to become linear enough for
practical use. That's why you will never see a grounded cathode triode,
feedback-less amp, at least not one that performs well. In grounded
grid, instead, you get that heavy negative feedback "automagically".
Power MOSFETs don't lend themselves very well to grounded gate
operation, so when using MOSFETs in a linear amp it's necessary to use
explicitly implemented negative feedback.
The good news is that with modern MOSFETs having rather high gain at HF,
and legal limit ham amps requiring only 12-13dB of actual gain, there is
a reasonable gain headroom to implement some worthwhile negative feedback.
Jerry,
I am continuing my learning curve to be tech-saavy and would love to
build a 50V SS amp instead of playing with HV supplies and ceramic
tetrodes. Where does one start?
First, look at schematics of typical amplifier stages. The old Motorola
datasheets and application notes about the MRF series of power MOSFETs
provide lots of useful information. So do those by APT, Ixys, ST and
various other manufacturers.
Then, question everything you just read, because some of those
application notes contain gross mistakes!
Then, think out of the box: The classical push-pull circuit with
grounded sources shown in most of those papers is by no means the only
way to do a broadband amplifier. This is actually the most important
step: THINK! Don't just copy what others have done decades ago.
Then set up an experimenter's bench: Get a bunch of cheap small MOSFETs,
with which you can experiment at your leisure. Don't try to build a
1.5kW amp as your first ever MOSFET project - that route would only lead
to frustration and money loss. Instead make some small amplifiers, that
produce power outputs from about 10 to 30 watts, when driven by 1 watt
or so. The MOSFETs can be IRF510, for example, but a great many other
cheap, relatively low power, TO-220 cased ones work well, at least on
the lower HF bands. As power supply, 13.8V is fine, and is available in
most shacks. As driver, use your HF radio, with a good attenuator. That
protects the radio from high SWR, and gives you the small power you need
to drive your educational amplifiers - because that's what they will be.
The output of your amps can be consumed in a modest 50 ohm dummy load.
Don't put it into an antenna until you are sure it's clean.
You will need test equipment. If you don't have one already, do get an
oscilloscope. It's absolutely essential in any sort of RF work. It can
be a used one - many are available at rather attractive prices. For
testing amplifiers for distortion, you also need a two-tone generator
and a spectrum analyzer. The two-tone generator is free: Use some sound
generation software for the PC, and feed the sound card output to the
radio's mic input. Or for more comfort, generate a loopback audio file
of the two-tone signal, put it into a portable MP3 player, those little
cheap pendrive-like thingies, and there you have a portable,
self-contained two-tone generator!
For a spectrum analyzer good enough to test an amp for IMD, you just
need two things: Any totally bare bones, basic, simple, trivial
receiver, with the only requirement that it should have enough dynamic
range (60dB is fine for this purpose, and that's easy to achieve), and
some software. Here are two ways to do that:
1) Use an USB-controlled DDS signal source, which can be bought as a
ready made board, and combine it with a simple mixer circuit and George
Steber's spectrum analyzer software. That gives you an HF+ spectrum
analyzer with about 80dB of dynamic range, for a rather small price.
2) Use an existing SSB receiver, that has enough bandwidth, at least
about 6kHz. Many radios, like my TS-450, allow using the FM filters in
SSB, achieving enough bandwidth for this purpose. Feed the audio output
into the computer, and use software like Spectrum Lab. This can be a
100% free solution, but might be a bit challenged in bandwidth and
dynamic range. For basic IMD testing it's still OK.
Now build amplifiers, modify them, experiment what happens with
different transformer configurations, types, different kinds of
feedback, varying bias levels, and so on. Once you have reached the
point that you fully understand how those little amps work, you can
start to design and build a big one. The experimentation might end up
with a few burned MOSFETs, but at 1 dollar each, that's no catastrophe.
You should instead try not to burn the big ones. Depending on the kind
of circuit used, the complete set of MOSFETs for a legal limit amp can
cost you anywhere from about 70 to 2000 dollars! It goes without saying
that the 70 dollar option is more attractive to me and to most hams! ;-)
In the course of your playing with small, cheap MOSFETs, make sure you
achieve a complete understanding of the cooling issues, because these
become critical in big amps. You need to be able to calculate how hot
the silicon chip inside a MOSFET's case will be, when the air is at
normal room temperature, considering the MOSFET's characteristics, those
of your heatsink, the way you are using it, how you mounted the MOSFET,
and of course its operating conditions and thus the amount of heat it is
producing.
And if you have questions that are not answered in those papers and
other literature you can find, ask! I'm one guy who loves answering such
questions, and I'm certainly not the only one.
Manfred
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Visit my hobby homepage!
http://ludens.cl
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