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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