Geert,
I would also look at other devices. For instance, NXP produces a
number of devices (like the BLF578XRS) where a single device produces
up to 1400 watts, up to 500 MHz.
I would absolutely love to have a few of these to play with - but I
would certainly not attempt to build a legal limit HF amplifier using
just one of them! Cooling them at that level is an impossible mission.
The selling factor is that they are extremely rugged and are spec'ed
to survive open antenna or a short without any damage.
Yes - but only as long as the junctions stay below a certain critical
temperature! At 1400W in class AB, that means pulsed service only.
In a conventional linear amplifier, operating in class AB, efficiency
cannot be expected to always be higher than 50%. So the old rule remains
valid, that about as much power is being dissipated, as is being sent to
the RF output. But at 1400W you would need to maintain the mounting
surface of this transistor below 46 degrees Celsius, and that's mission
impossible even with water cooling, considering that the mounting
surface is just 32x10mm.
But two of these devices could comfortably deliver legal limit power.
I don't know what the IMD characteristics are. Anybody tested these?
Gain is rated as 23.5dB at 225MHz, in what seems to be saturated
service. At HF and without entering saturation, I would not be surprised
to see something like 40dB power gain, or even more. That means that you
can design an amplifier providing the typical 13dB gain needed in ham
service, while having strong negative feedback. That should result in
pretty good IMD.
Two modules, each using one of these and designed for 750W, combined,
should work great and be reasonably easy to heatsink (the emphasis is on
"reasonably", not on "easy"!). But at about 300 dollars each, I'll pass.
Using many small ones is far cheaper - and cooling gets easier!
Such very high power RF transistors are mainly attractive for high
efficiency and pulsed amplifiers. For common class AB linear amps, we
need transistors that have enough brute power dissipation capability,
for their electrical capabilities.
Cooling of solid state power devices is not something the designer can
add almost as an afterthought, once the electrical design has been done.
Instead, it is the prime, central thing! A linear power amplifier stage
is basically a big valve: It regulates the flow of power from the power
supply to the RF output terminal, by regulating the instant voltage at
that terminal - and dissipating as heat all the surplus power that
arises from the product of the voltage not needed at that instant
(supply voltage minus output voltage) and the current flowing. So, the
concept of a linear power amplifier is that big valve-like device,
combined with all the necessary heat sink, fans, water, radiator, etc
coling stuff around it - and then, almost as an afterthought, you add
the rest of the electronic circuit! :-)
Anyone who wants to go the other route - make a clever circuit, then add
a little cooling - needs to make a high efficiency amplifier, not a
plain class AB one. And that, my dear friends, is a non-trivial task, if
we want to achieve good linearity, simple, broadband operation, and
reliability. We can save most of the heatsinking, almost half of the DC
input power, and a good deal of size and weight, but we have to invest
some real brains and time in it - and those two assets are rarest
nowadays, it seems, including here at my lab!
Manfred
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