To add to Jim's comments, what about all the stand alone devices coming out
these days?
I have been working on one of W6PQL's 2 meter jobs using the Freescale
devices or the BLF578XR; BLF578XRS. The bottom line is always cooling the
device regardless of what it may be. Getting that waste heat out of there
can be a problem. However, I think it can still be done in much less area
with a SSPA.
Paul
WD8OSU
On Fri, Oct 17, 2014 at 4:22 PM, Jim Garland <4cx250b@miamioh.edu> wrote:
> 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
> > ========================
> > _______________________________________________
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> > Amps@contesting.com
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>
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