Manfred,
Love the thinking-outside-the-box!
Has someone patented this who might block production?
Any concerns about potential RFI from a switching-amp
in a rf-amp circuit?
Has anyone tried this before?
Are you considering a crowd-funding proposal to raise
funds for the project?
David KD4E
> 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
--
David Colburn, KD4E - Nevils, Georgia USA
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Restored to design-spec at Heaven's gate 1Cor15:22
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