I am surprised that in all of this now one has mentioned the class E topology.
For CW only use it is perfectly adequate and can be very inexpensive.
There are parts available now with switching times adequate for 30Mhz that are
also high voltage (500-1kv).
This means that it would be possible to rectify and filter the house supply and
have full legal limit with about
12-15W of drive. No transformer!. We are using this approach at the moment to
pump a CO2 laser at 40Mhz. The cooling
requirements are minimal and the parts are reasonable in small quantities
($79.00). We are seeing 87% efficiency (counting
the drive power) at the moment.
The downsides are:
Unlike other semiconductor amps, class E is not broad banded, however tuning is
minimal at the
higher frequencies. It will require and input and output tuning network for
each band, like g-g. The tuning is broader than most g-g
amps that I have messed with, but it is still required. The tuning components
don't need the withstanding voltage ratings of most tube
amps.
If you want line isolation (we don't need it with the laser), you need an
isolation transformer.
In class E output power is a function of supply voltage. If varying the output
power is required, you need a capable variac. These can get really expensive.
The gates of MOSFETS are incredibly easy to punch through with too much voltage
(20V for most). I have busted a couple
with my 706MKii on its lowest power setting by notching up the power just a wee
bit. Strict, severe ALC would be a good idea.
Of course the drains are sensitive to the voltage soaring that occurs with
mismatch. VSWR protection would also be a must.
The parts are found at:
http://www.dirnrg.com/index.html
Look at their power MOSFET section.
A really worthwhile design article is found in Communications Quarterly. ( I
forget the issue, sorry(: ). The articles in QST by the Rutledge crew are also
worthwhile.
Fire away boys,
George K. Watson
K0IW
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