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Pete es Co
The class C tube amplifer is the nearest you can get to
class D or E and well designed would produce 70% efficiency.
The class D or E is not really an amplifier in the
convential sense it is merely an RF switch at the frequency of interest and the
voltage generated is increased by a step up transformer with probably 80% to 90%
efficiency if you are lucky.
Like some have said a tube amp is more robust and tolerant
of mismatch to antenna without smoke. Fets are good but can go bang if one is
not careful about matching to load.
I have some Dentron amps for the HF bands and they have
been going for years.
Fets are probably more manageable, compact and safer
regarding voltages, beware of tubes with voltages of a few K/volts at high
current, like an electric chair!!
73 de mal/g3kev
----- Original Message -----
Sent: Tuesday, June 12, 2012 11:15
AM
Subject: Re: LF: RE: Class E etc
Hi Ken
I believe you can
operate some thermionic devices in higher classes than C but they
generally lack the frequency response of a modern FET. I won't go into figures but just think about it. The high
efficiency output stage has to operate
like a switch it is not actually an amplifier. The faster it changes state
from fully on with ultra low resistance to fully off (which means a
device that will operate well into the VHF) the higher the
efficiency. If you have a device that is barely rated to say a couple of megs
its not going to be much good (in terms of what we are talking about) at
500kc/s. Remember you are trying to get the magnetic field in the inductor to
rise and collapse at a very fast
rate in-order to generate a
high electromagnetic field at the drive frequency. We are not talking sine
waves here as a scope on the drain will show. Although a scope on the output
side of the drain inductor will show quite a clean looking sine wave. Try
it.
That chart at the end of that PDF file is very clever and is the guide I use to
bring a PA into optimum performance.
What I was trying to say is that the math is all very well but if
you are trying to make a practical and efficient CW TX for LF
then there is enough empirical
data from following the guide lines lines shown on the sites I have
posted that will get you a good TX. If you want a doctorate in
theoretical physics to bamboozle you
colleagues then that will detract
from the time it takes to build a very interesting high power, efficient TX
design well within anybodies capabilities. The GW3UEP design being
IMHO a very good (Scalable) example with no RadCom style unobtainium in the components listing.
73 es GL Pete M0FMT IO91 UX
Hi
Pete.
About 40 years ago my
integration was quite good, liked doing 2nd order differential equations, partial
integration and integration by parts, the one thing I had big problems with
was 3 dimensional integration of a point in free space . Looking
at some of the sites my Class C MOSFET
PA is probably more like Class E by default. The reason I like valves is
because they are very forgiving devices unlike semiconductors, also the keep
the shack and the cat warm.
73s
Ken
M0KHW
Ha ha Stefan, how
good is your Integration Ken?
Here are a couple
of sites that should answer your questions. You need a good dummy load and a
good oscilloscope. Setting the conditions for the amplifier is to do
with A/ the correct driver preferably using a FET drive chip (like TC4422 non and TC4421 inverting)
that will turn the FET fully on
and fully off on each cycle. Very fast rise time with accurate on off
timing (mark / space). Ensuring the max permitted gate voltage for the
FET is not exceeded otherwise they
pop, look out for transients, a diode clamp helps. B/ Setting the shunt
capacitance and inductance in the drain circuit can be done empirically See
article below. Then follow it with a low pass filter, although the
output wave will be surprisingly
clean. Calculating the values is difficult as you don't necessarily know
all the starting parameters and in any case the result is going
to wide of the mark. The site below will gives some starting values. Best to
use 'Scope measurements to
set the output conditions. You are looking for fast, very fast On / Off rise
times. Gate drive and understanding the drain circuit is the
key.
These amplifiers
are capable of incredibly good efficiency well above a dodgy class C amp. This
means the heat dissipation from the FET
is very low at high power giving cool heat-sinks!
The GW3UEP
(do a web search) single ended design is capable good efficiency and
high power depending upon the FET used
and Drain supply voltage. I have modified one to run reliably at 140watts and
is capable of almost twice that into a dummy load with a heat sink that is
fairly cool. Adjusting L and C empirically using
the 'scope to get the correct drain wave shape for max efficiency is the way
to go. The article below shows you what you are looking for. I set my TX up
into a 50 Ohm dummy load adjusting L and C to get the correct drain wave shape
on the 'scope then measuring the peak voltage across the Dummy load. I
then transfer my connector from the Dummy load to the input of the tuned
transformer in the antenna up-lead. I adjust
the coupling link to give the same peak voltage across the tuner link
turns. You are not looking for maximum peak just the sames as across the dummy
load indicating a 50ohm match. This is done by either adding or
reducing the number of turns (about two turns) along with bringing the
the transformer to resonance by measuring the peak up-lead current,
here you are looking for the maximum
peak.
Oh yes and WIMA high voltage high value capacitors from
Maplin work
well.
Read the articles
and search info on class E amplifiers.
For a good
practical circuit for a single ended amp that is a scalable surefire
design, no need to reinvent the wheel.
For antennas and
antenna feed methods a "look no where else site"
73 es GL Pete M0FMT IO91UX
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