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Triggertron and a delay line (lumped) modern spark
set hi
Sent: Tuesday, June 12, 2012 12:15 PM
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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