When I joined this list a few years ago, it was in search of information
and perhaps help regarding an envelope-restored amp I was trying to
develop. It's funny that only now I learn about the Mini Lini of 2005!
I shelved my project several months after joining this list, even though
I still have the prototype in a box somewhere. I ran into a stupid
little problem I couldn't solve, and I have a feeling that SGC ran into
the same problem, and that this was why they never sold (or even got FCC
approval) for the Mini Lini.
In my prototype, I was using a linear modulator (accepting the
inefficiency), as an interim step while testing the rest of the concept,
and so the differential delay of phase and amplitude wasn't such a big
problem. Nevertheless, I obtained a lousy IMD performance. The reason
was an often overlooked characteristic of MOSFETs: The reverse transfer
capacitance rises very much when the drain voltage lowers. With bipolar
transistors much the same happens. And this creates a big problem: Since
class E (or C) amps need to be driven into saturation, there is a
relatively large drive power available, and when the drain voltage goes
low, the capacitance goes high, the power capacitively coupled through
the transistor wins over the amplified power! And since the two are in
opposite phase, a funny looking distortion happens: Instead of the clean
straight zero crossings on the envelope that are normal for a clean two
tone signal, there are now TWO zero crossings, with a small sine between
them, in which the RF has reversed phase. Spectrally, this manifests
itself as very strong IMD, and when listening to the signal from this
amp, it is audibly distorted.
Using transistors, I see no good way around this. Only applying some
predistortion, so that when the original amplitude reaches zero, the
drain voltage sreaches the level where coupled-through and amplified RF
just cancel out. But this is frequency sensitive, load-sensitive,
tuning-sensitive, and even heat-sensitive! It's a nightmare.
Perhaps someone cleverer than I am can see a better solution. If not, I
will have to clever-up and invent some adaptive-corrective feedback system.
A Brazilian ham published an article in QEX some years ago, about such
an amplifier, but he used tubes, which are a lot easier in this regard.
Even so, he also must have had problems in the small-signal area,
because he ran the amp in envelope restored class E only from a certain
amplitude level upwards, and below that he froze the supply voltage and
let the amp run in conventional linear mode. I don't trust the IMD can
have been very brilliant that way.
So, I definitely think that the connector issue cited by SGC as the
reason for indefinitely delaying the Mini Lini was just an excuse. The
real problem lies deeper inside.
And to compensate for the delay of the switching modulator, well, let's
just use a roll of coax cable as a delay line for the RF drive! With a
250kHz switching frequency and all the filters and delays there, just a
few kilometers of cable would be needed! There goes the "mini"!
So, my project of a high efficiency small solid state legal limit linear
amp has reverted to the ages old class D concept. I just need some
MOSFETs rated at several hundred volts, able to take 10 amperes or so,
that can switch in about one nanosecond. Hey, semiconductor
manufacturers, what are you waiting for?
Oh, well, I think I could settle for 5 nanoseconds! Just would have to
drop 6 meter coverage, and accept some more spurs! ;-)
Manfred the optimist.
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Visit my hobby homepage!
http://ludens.cl
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