Hi Manfred,
<snip>
> 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.
I have a PDF somewhere that addresses this problem. I'll dig it out and send
it to you
>
> 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.
I once looked at doing it that way too -- gets a mite ugly.
I'm currently building a test bed final with a 6146 in Class C, to be driven
by a homebrew DSP exciter, that should be good for 250W PEP out.
<snip>
>
> 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! ;-)
Look at Directed Energy, now owned by IXYS:
http://www.ixyscolorado.com/
73 & Good morning,
Marv WC6W
http://wc6w.50webs.com/
> Manfred the optimist.
>
> ========================
> Visit my hobby homepage!
> http://ludens.cl
> ========================
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|