Manfred, I didnt become proficient in hollow state amplifier design by not
extensively studying, and experimenting with linear circuits that were as
strange in the 50-60's as SS is to many now.
OTOH there were enough old design tubes available to place a somewhat clean
SSB signal on the air until tube manufacturers got up to speed and equipment
manufacturers started using them and/or developed FB circuits for the 6146.
SS QRO development has been slow and commercial user requirements have
driven it, HF is a small market these days and the money is at VHF and up.
Ive no problem with 12-15dB gain when 100-200W of drive is available, just
think of it as GG SS (-;
My own R&D experience has all been at 10 gHz and much higher, mostly
commercial and military but not at QRO levels!
Your special FET might not be that far off the way developments are going, a
new startup may already be working on it.
Carl
KM1H
----- Original Message -----
From: "Manfred Mornhinweg" <manfred@ludens.cl>
To: <amps@contesting.com>
Sent: Tuesday, December 02, 2014 3:06 PM
Subject: Re: [Amps] Old Moto RF Decks
Carl,
Now when is Freescale going to offer a 2-30 or 2-50 mHz device that has
high efficiency and very low IMD at the same time and be targeted at the
linear SSB amp market?
Freescale provides you the best FETs they can. It's up to _you_ to come up
with an amplifier circuit that delivers that high efficiency and
linearity!
Expecting to take a device and connect just the drive signal, power
supply, output line, and maybe a bias source, to get a clean output at
high efficiency, unfortunately exceeds the possibilities of _any_ active
device, be it solid state or empty state!
A tube can be linearized to some extent by adding additional grids, and
cleverly shaping the electrodes. But it still needs lots of idle current
to provide low IMD, and thus will have poor efficiency. One could do
something similar with FETs, by adding a lot of source ballast resistance.
That costs in efficiency and in gain, but does improve linearity a whole
lot. Instead by using a plain device and building a clever circuit around
it, you can get _both_ high efficiency and good linearity. At least in
principle. It can get complicated, though.
If you want to keep things simple, the easiest way to reduce IMD in solid
state amps is to give them lots of negative feedback. To do that, you have
to start with excess gain! 30 years ago that was somewhat hard to do. But
nowadays there are FETs that have huge power gain in the HF range. Given
that the typical ham needs an amp having just 12dB gain, you can burn up
the FET's excess gain via negative feedback, getting improved linearity in
a class AB amplifier.
But the efficiency will still be modest. If you want 80-95% efficiency
along with good linearity, it's time to look into all the complexity of
class E amplifiers with modulated switching power supplies, and a DSP box
wrapped around. And at that point you don't really need a FET with any
particularly good "natural" IMD spec! You just need an RF-capable switch.
So, your teasing Freescale to give you a super-linear, efficient FET with
two coax connectors and a power plug on it, will hardly lead to any
success! It lies in the nature of the thing... ;-)
I can't deny, though, that I would like such a special FET myself.
Manfred
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Visit my hobby homepage!
http://ludens.cl
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