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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