A fascinating writeup, Manfred. Thanks! Wonder if you could elaborate or
provide a referene on "... we need to place our RF power transistors in half
bridge or full bridge configurations, with effective antiparallel diodes."
I'm unfamiliar with that design. Also, I wonder if a CW-only amplifier using
switch mode techniques would be significantly easier to implement than a
purely linear design?
73.
Jim W8ZR
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Manfred
Mornhinweg
> Sent: Friday, October 17, 2014 12:24 PM
> To: amps@contesting.com
> Subject: [Amps] Solid State Amps
>
> Dear all,
>
> Dan touched the subject of solid state amps, and Louis was quick to state
that
> most hams would prefer a good tube amp. If you ask me, the
performance/cost
> ratio will dictate what hams finally prefer, rather than any philosophical
concepts.
>
> So, what we need to finally move tubes out of the ham realm (except for
those
> who really love tubes, of course), is making solid state amplifiers that
are
> better and less expensive than tube amplifiers.
>
> And the best approach to do that is _not_ by porting tube era technology
to
> solid state devices, nor is it to keep building copies of Helge Granberg's
> designs forever. These approaches simply produce a poor performance/cost
ratio,
> when taken to the 1500W level.
>
> Let's see what the weaknesses of solid state amps are:
>
> - Heat. Solid state devices simply are very small, and don't tolerate
extreme
> temperature. So, a high power, class AB, solid state amplifier will ALWAYS
be
> problematic in terms of cooling. It will need large heatsinks, fans, heat
> spreaders, and careful design of the thermal aspects, just to start
becoming
> viable.
>
> - Fragility: RF power transistors are usually run very close to their
absolute
> maximum voltage spec, close to their maximum current spec, and at or even
above
> their rated thermal capability, with the heat sink system used. Any
problem like
> non-perfect SWR, relay glitches, etc, and their survival depends 100% on
> excellent protection circuitry. Tubes instead are so forgiving that in
practice
> they don't need protection circuits in most cases, or some tubes need
simple
> circuitry to protect against excessive screen or grid dissipation, but not
much
> else.
>
> - Poor linearity: Both bipolar and field effect transistors are less
linear than
> tetrodes and pentodes, and while better than triodes, they don't have
enough
> gain to use them in grounded base/gate configuration. So, they depend on
> negative feedback or other external means, to arrive at good IMD specs.
Many
> designers still don't grasp this concept well enough, and try building
solid
> state class AB amplifiers without negative feedback, getting horrible IMD
> performance.
>
> Now some people have tried, and are still trying, to solve these problems
by
> brute force methods: Use lots of transistors, on big heatsinks, run them
well
> below their maximum specs, use UHF transistors at HF to get enough gain
that
> allows using lots of negative feedback, and put in complicate protection
> circuits. The results of these efforts can work reasonably well, producing
> amplifiers that are instant-on, no-tune, reliable, and about as large and
heavy
> as tube amplifiers - but the solid state ones tend to be more expensive,
done
> that way. And often the implementations are simply wrong and unsafe, for
example
> by relying on an SWR sensor placed between the low pass filters and the
antenna.
>
> What we need to do, my dear friends, is something totally different. For
> starters: Forget class AB, because it's too inefficient, and forget
Granberg's
> push-pull configuration, because it has no inherent protection features
and
> needs problematic transformers.
>
> Instead of Granberg's design, we need to place our RF power transistors in
half
> bridge or full bridge configurations, with effective antiparallel diodes.
This
> configuration eliminates all risk from overvoltage. Then we need to run
our
> transistors in switchmode, _not_ in any linear mode, to get rid of the
heat that
> causes so much trouble. Then we add simple current sensing with quick
shutdown,
> to protect against severe overcurrent situations. We need to take the
highest
> voltage transistors we can, up to a level of 400V or so, to get rid of the
ultra
> low impedances that result from low voltage operation, and which are hard
to
> handle. And instead of a broadband transformer (not very easy at the
kilowatt
> level), followed by relay-switched low pass filters, we should use
> relay-switched resonant matching networks. That's no more complex than the
low
> pass filters, and the resulting Q is low enough to pre-tune these networks
to
> each band and then forget them.
>
> And then, of course we need to add circuitry around the amplifier block,
to
> obtain a linear transfer function despite the switching operation of the
RF
> transistors. This can be done by RF pulse width modulation of the drive
signal,
> power supply modulation, bias modulation, a combination of two or three of
> these, or any other method. This is far more complicate than a traditional
tube
> amplifier, of course, but it uses cheap, small, widely available
components, and
> so it's inexpensive to implement.
>
> The result would be an instant-on, no-tune, small, lightweight, silent,
highly
> efficient, reliable _and_ inexpensive legal limit amplifier.
>
> Anyone actually developing this concept to market maturity can put all
existing
> ham amplifier manufacturers out of business. A scaring thought - for them!
>
> Do you notice the logic in this? Going from class AB to a switching mode
> achieves several important advantages:
>
> - Cooling becomes very much simpler, cheaper, and silent.
> - Power supply requirements are drastically cut down, producing advantages
in
> cost, size, weight, etc. A 1700W power supply can power a 1500W amplifier.
> - Power consumption is reduced a lot, an important selling point in many
> countries that have expensive electricity. Maybe not in the US, where it
is
> almost free.
> - The transistors needed are very much smaller and cheaper than those
needed for
> class AB, due to low dissipation requirements.
> - A good active linearization circuit can produce far better linearity
than
> class AB with 10dB of negative feedback, and even better than that of
tetrodes.
>
> And the difficulties involved in this approach:
>
> - Finding ways to get around the limitations of present-day RF power
> transistors, in terms of voltage-dependent internal capacitances, slew
rate
> limitations, and high voltage handling.
> - Summonning the determination to do all the detail design work, and break
free
> from the idea "if Granberg did it that way, that must be the best/only
way".
>
> Any idea, anyone?
>
> Maybe we should start a collaborative open project, developing this thing!
The
> final goal: A solid state amplifier no larger nor heavier than a typical
HF
> radio, that can produce solid legal limit output in all modes, with no
time
> limit, with good IMD performance and high reliability, a total parts cost
around
> $500, and selling to those who are too lazy to build it, for around $1000.
>
> I'm just waiting for the right transistors to show up, and then I will do
it
> myself. With the transistors I know right now, I would get up to the 40m
band
> only, or at most to 20m, but not to 10.
>
> Manfred
>
> ========================
> 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
|