[Amps] Future of RF AMPS

[Richard Solomon]

M2 markets both a 6 and 2 Meter Monoband Amp, 1250 watts out,
uses a single device. Granted there's a time limit on AM and JT modes,
but there are no limits on SSB.

>From what I read, the problem is more in the Output Circuit design than
cooling for an HF model.

Look at the W6PQL site for info on cooling one of these LDMOS devices.

73, Dick, W1KSZ


On Fri, Sep 13, 2013 at 1:39 AM, Jim Thomson <jim.thom at telus.net> wrote:

> Date: Wed, 11 Sep 2013 22:36:45 +0000
> From: Manfred Mornhinweg <manfred at ludens.cl>
> To: amps at contesting.com
> Subject: Re: [Amps] Future of RF AMPS
>
>
> 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
>
> ## What about these freescale devices ? That company in OH land manages to
> get 800w to 1 kw out of just one device.   I saw on U tube,  1 of em being
> used in
> a 1250w CCS  FM broadcast TX, 88-108 mhz variety.  Also on U tube, was a
> 2.5 kw CCS eme  144 mhz amp... using 2 of em.   The cooling used was
> unique on
> all 3 of them.
>
> ##  On a similar note,  a buddy built a dummy load using those 50 ohm flat
> pack
> resistors, looked like aprx  25 x 50mm each.  He mounted 9 of em on a big
> AL
> heatsink,  3 in series to make an assy.....and 3 x identical assys.  Each
> assy is 150 ohms.
> Then the 3 x 150 ohms are all in parallel to make 50 ohms.  They are rated
> at 800w CCS each.
> The air cooling was not up to snuff..and one exploded.  Plan B was to
> lower the fins face down
> into a tub of ice water..problem solved.    + 69dbm was being used at the
> time.
>
> http://groups.yahoo.com/neo/groups/ham_amplifiers/photos/albums/654737392/lightbox/543825280
>
> Later... Jim  VE7RF
>
>
>
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