[Amps] Solid State Amps

[Manfred Mornhinweg]

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

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