Hi Manfred,
You might consider a simple envelope tracking scheme to reduce the
dissipation in AB. Just keep the VDD high enough to output the
applied signal and perhaps idle at 1/4 the normal VDD/dissipation.
Rather than liquid cooling, have you seen the little heatpipe
heatsink assemblies which are used for CPU's in the latest computers?
They can fit one transistor each and dissipate a few hundred watts
I considered a scheme to take the audio from the input of a standard
transceiver and use that to run the modulator for an amplifier in
Class AB/D. The RF delay is long enough but, all the filters in a
typical transceiver play havoc with the group delay. The DSP procesing
necessary to "auto-equalize" an existing transceiver is in excess of
that required to make a transmitter from scratch.
Have you seen the Directed Energy (DEI) parts from IXYS? They are
their best though pricey!
73 & Good morning,
Marv WC6W
http://wc6w.50webs.com/
-- Manfred Mornhinweg <mmornhin@gmx.net> wrote:
Hi all!
<snip>
Marv:
> There is a small problem, often overlooked, in the envelope
> restored Class D/E/F "linear" scheme. The group delay through the
> modulating path must be equal to the RF path delay for the system to
> exhibit low distortion. This can be realized over a narrowband but,
> when a wideband system is considered it may prove to be easier to
> build an entire envelope restored transmitter than to construct a
> standalone linear.
Well, my self-inflicted task now is to make a noiseless legal limit
linear amplifier driven from existing 100 Watt radios, so building a
whole transmitter is not something I'm considering now. If the AB/F
scheme turns out to be excessively troublesome, I will probably revert
to a classic class AB design and beef up the thermal design. Less
elegant, but almost guaranteed to work well. And just in case if
MOSFETs
turn out troublesome, I have two trusty 4CX1500B tubes lying around!
But
they are not suitable for silent (blower-less) use.
In any case, in his QEX article about his AB/F amplifier, Saulo
included
screen shots of the dual tone waveform and the spectral analysis. There
is indeed a plainly visible distortion caused by the delay in the power
supply response, and this reflects in the spectrogram as slightly
raised
IMD. But as Saulo points out, the IMD is still very acceptable,
actually
it's better than some commercial AB amplifiers! So it seems that this
AB/F approach without controlled delay is "good enough", even if it is
clearly not perfect.
My intention is to make the AB to F transition at the lowest possible
power level. Saulo made it at about 25% of the amplitude. I intend to
go
lower, as low as the driving of the MOSFETs will allow. That should
reduce distortion. Also I was toying with the idea of adding a delay
line to the RF drive, but that might be simply not worth the effort.
When you see how many hams are using their amps without any ALC,
happily
flattopping away with the IMD less than 12 dB below the output, it
should become clear that an imperfect AB/F amplifier with the IMD at
-28dB should be quite acceptable!
But then, the "recently discovered" IxysRF devices are rated for stable
linear operation at 150 Volt, so one of my reasons to use class F and
envelope restoration is weakening... Maybe I go the brute force route
of
pure class AB! It depends on the outcome of the experiments. After all,
when anyway using a SMPS, the physical difference between a class AB
and
an amplitude-restored AB/F is just in the small components and in the
output network! It's easy to change along the road.
I have gotten lots of info to think about. Thanks to all of you, and
keep it coming!
Manfred, XQ2FOD.
----------------------------
Visit my hobby website!
http://ludens.cl
----------------------------
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