[Amps] Ampleon BLF189XR 1900W LDMOS transistor

[Kevin]

W6PQL.com

He originally started using the LDMOS devices on 6m and up and then 
built the HF SSPA. I believe the schematics are in the article. I know 
the spectrum shots are, both before and after adding his LPA. He uses a 
Chebyshev design in the LPA, 5 poles, and hits FCC harmonic requirements 
160-6m. He has built a two device amp for someone using two BLF188XR's. 
His filter is rated at 1500W. I'd feel better if it was 2K or higher.

An LPA design I saw in the handbook used a Cauer architecture for the 
filter. The capacitor values are critical and they are expensive. You 
also need to tune the filter to land the nulls right on top of the odd 
harmonics.

The LDMOS devices are designed to tolerate 65:1 SWR, 3:1 probably 
wouldn't be a problem so where is the need for a tuner? If you're trying 
to load a wet string with a tube amp your going to need an outboard 
tuner so why not use it with an SSPA? The auto tuner argument is a straw 
man argument.

These LDMOS devices are NOT like your fathers Bipolar and JFET, or even 
more modern MOSFET devices. They won't pop when facing a 2:1 SWR and 
have amazing amounts of gain at HF.

They do require copious amounts of clean DC, 50V, 60-65A for 1500W.


On 10/20/2016 9:55 AM, Manfred Mornhinweg wrote:
> Kevin, Jim, and all,
>
>> W6PQL tested the NXP BLF188XR, the device he uses in his 1KW amps,
>> and came up with 1100W before hitting 1dB compression and it's a
>> 1400W "peak" device.
>
> Is there a web page or something where we can see what exact circuit he
> used? I ask this because the 1dB compression point isn't a fixed
> characteristic of a given transistor, but rather one involving the
> transistor, the rest of the circuit, the frequency, and the supply
> voltage. It makes no sense at all to say "the BLF188XR has 1dB
> compression at 1100W". Instead it makes a lot of sense to say "this
> particular amplifier circuit, using a BLF188XR, and a 50V supply, has
> 1dB compression at 1100W and 30MHz", or something like that.
>
> To answer the question of whether to use one or two devices for a given
> set of conditions, it goes roughly like this: First you find out what
> supply voltage, load impedance, and amount of feedback you need to
> achieve the desired power at 1dB compression, at the highest frequency
> of interest, using a single device. Then you calculate how much power it
> would dissipate, under those conditions. Then you see whether you can
> reasonably build a heatsinking system that would make the device run at
> a safe junction temperature (providing the desired minimum life span).
> If you can, you may go with a single device. If you cannot, you use two
> of them, which makes the thermal handling VERY MUCH easier.
>
> Even if in some situation you might just be able to use a single device,
> it can be better economy to use two of them. That happens when two
> devices on a simple, smallish heatsink come cheaper than a single device
> on a super duper peak performance watercooled copper block, radiator,
> pump, etc.
>
>> If I was building one with the BLF189XR I'd still use two devices.
>
> Me too, probably, for thermal reasons, but first I need to see the
> datasheet. And if using two, I would use each of them in a complete
> amplifier block, and then combine the outputs. The more obvious solution
> of making a single push-pull amplifier with one complete device per side
> is very hard to implement properly, because of the extremely low
> impedance. Also making two amplifier blocks allows physically separating
> them by a significant distance, allowing much easier cooling. For
> example, you can use one of those heatsinks that form an air duct,
> having the fins inside, with a single fan blowing through them, and
> mount one amplifier block on each side of that heatsink. That's much
> better than mounting them side-by-side on a flat heatsink.
>
>> The big question is the low pass filter. The single most expensive
>> part of an SSPA.
>
> Yes.
>
>> ### This is my problem with these SS amps, the LP filter.  And you
>> require a myriad of them, not just one of em.   On a NINE band ham
>> amp, you would require a bare minimum of SEVEN 1.5 kw CCS rated LP
>> filters... AND you have to be able to switch them in / out.
>
> The coils are easy enough. Switching can be handled by inexpensive
> relays. The problem are the capacitors. Factory-made capacitors with
> adequate ratings are rare and expensive. Like a 180pF capacitor rated
> for 15A of RF current, along with being small enough to have an
> equivalent series inductance of only a few nanohenries. Very few
> capacitors meet such specs, and they tend to be expensive.
>
> I have been considering making a whole legal limit low pass filter board
>  based on homebrew capacitors. Metal-clad multilayer capacitors are very
> easy to make from copper foil and some dielectric. The dielectric
> can be either mica sheet or teflon film. Each has its own plusses and
> minuses. Teflon is soft, so one has to be careful with burrs in the
> metal punching through it. Mica comes in uneven thickness, so that it
> takes some cut-and-try to achieve the correct capacitances. Mica has a
> higher dielectric constant, so the capacitors end up being smaller. But
> even with the larger teflon caps, the series inductance is very low,
> because of the wide and short conductors that result from the 
> construction of metal-clad capacitors.
>
> If anybody has a good idea what other dielectric material could be 
> used, that's easy to get, please let us know! It needs to be 
> heat-resistant and have a very low dielectric loss, of course. A 
> somewhat highish dielectric constant would be a plus.
>
>> You dont  require ANY LP filters on any tube amp.
>>
>> ##  You also forgot the SS amps also require the fully automatic,
>> 1.5 kw CCS rated antenna tuner.  Again, a tube amp doesnt require a
>> tuner.
>
> Well, I would like to remind everybody that there is no law requiring
> that solid state amps must be broadband or autotuned! In the ham world
> it's just a custom that most tube amps are manually tuned, with a few
> being autotuned, while solid state amps are broadband, with some having
> an add-on or built-in autotuner. But there is no technical reason
> whatsover preventing us from building a solid state amp that has a
> high-Q tank like a tube amplifier! That tank would replace both the
> switched low pass filters and the autotuner. And the operating
> inconvenience would be exactly the same as that of a conventional tube 
> amp: You need to tune the amp when changing bands, or when making a 
> large frequency change.
>
> In practice you would build the typical broadband push pull amplifier
> block, or two of them plus a combiner, and pass its output through a
> tank circuit looking much like a tube amplifier's one, with a big,
> simple bandswitch, a large coil, and all. Only that instead of a low 
> capacitance, high voltage tuning capacitor, both variable capacitors 
> would be of the same kind, having the same ratings as the loading 
> capacitor of a tube amp.
>
> It would be a good idea to analyze the various possible tank circuit
> configurations, as maybe the Pi circuit isn't the best for this
> application. In any case it would boil down to two 
> front-panel-adjustable elements, plus a single band-switched element, 
> and would look much like a tube amp's output circuit.
>
>> A  SS amp will blow its brains out with swr.
>
> Not if a correctly designed protection circuit is used in combination
> with these "extra rugged" LDMOSFETs. Their ruggedness allows them to
> survive the few milliseconds the protection circuit takes to kick in,
> even at infinite SWR.
>
>> None of em will operate full power into a 2:1  swr.
>
> Not without tuning. But a tube wouldn't either. It's just that
> essentially all tube amps have tunable matching circuits included. As
> soon as you include them in a SS amp, it will handle SWR as well as a
> tube amp, and even better: Because SS amps always have protection
> circuitry, and tube amps often do not! A serious operator mistake can
> blow out a tube (and many of them cost a lot of money!), while a
> properly protected SS amp should be impossible to blow up.
>
> Then again, of course it's possible to include total SWR protection 
> circuitry in a tube amp, but most tube amps don't have it.
>
>> ##  Your typ  SS amp operates at 50% eff at best.
>
> It should be possible to get to 60% or so in linear class AB, with a
> well-done design. It's just that most SS amp designs are poor! And these
> UHF-capable LDMOSFETs open the doors to high-efficiency linear
> amplifiers that simply can't be built with tubes. Such amps have
> efficiencies ranging from 85 to 95%. This side of things NEEDS to be
> explored by hams, NOW! Almost all of us (myself certainly included) 
> seem to be too old and too lazy to get serious, sit down and do it!
>
>> So for a 1.5 kw CCS  rtty / FM signal, the heat sink would have to be
>> able to dissipate 1500 watts  CCS.
>
> Anybody serious about RTTY and FM operation should either build an amp
> dedicated to these modes, or one that can be switched into these modes.
> Because RTTY and FM don't require a _linear_ amplifier. They can
> perfectly well be done using these LDMOSFETs in saturated class AB.
> That's the mode most commonly used in the "typical application" circuits
> in the datasheets, and an efficiency above 80% can be obtained without
> any strange tricks or complex circuitry.
>
> A practical legal limit ham amp might simply switch the supply voltage
> according to mode. With the higher supply voltage (50V) it would run
> 1500W PEP with 1dB compression, at about 55% efficiency, for SSB, and
> with a lower voltage, roughly 40V, it would run the same 1500W but with
>  6dB compression or such, requiring higher drive power (but still far 
> less than 100W), and achieving over 80% efficiency, for RTTY and FM. 
> No other change than the supply voltage switching is needed, and 
> that's trivially easy to do when using a switchmode power supply.
>
> The low pass filters, or tank circuit, need to be able to provide enough
> attenuation of harmonics even when the amp operates in the nonlinear 
> mode. That's a point to watch.
>
> An amplifier operating in this way would dissipate around 800W average
> during 1.5kW PEP SSB transmission in linear class AB, and only around
> 400W during 1500W RTTY or FM transmission in the saturated class AB 
> mode. It might be possible to handle this with a single device, the 
> critical condition being in the SSB mode, not in continuous-duty RTTY!
>
>> ## Will  2 x BLF189XRs even do 1.5 kw pep with real low IMD ??
>
> The real question is: Can the BLF189XR, mounted on the best heatsink 
> system you can make, handle the heat resulting from such operation? To 
> calculate this, I first need the data sheet of the BLF189XR.
>
> Third IMD levels of -40dB are pretty easy to achieve with LDMOSFETs, as
> long as you use a good amount of negative feedback, and stay out of
> saturation. Many ham designs use too little negative feedback, because
> they copy datasheet circuits intended for nonlinear, saturated class AB.
> And of course, many hams try to milk the amps for the last watt, which
> means entering severe saturation and creating horrible levels of IMD.
>
> If you want third IMD levels much lower than -40dB, combined with good
> efficiency, techniques like predistortion or t least active feedback 
> must be used. At that point things get more difficult. But it's 
> pointless to go below -40dB in a ham amp, since the radios used for 
> driving them won't be as good. And no one on the air will notice, due 
> to QRM and QRN being much stronger than the residual IMD. With an SDR 
> transceiver using predistortion to drive a matched LDMOSFET amp, you 
> can get better than -50dB at full output, but the practical value of 
> this is debatable. It's probably useful only in rather rare cases, 
> when several hams live and operate very close together in distance and 
> frequency.
>
> I think that -35dB is a level good enough to use as a target value for
> 3rd IMD in general purpose linear amps. Most HF transceivers are
> significantly worse than that at 100W, specially on 10 meters.
>
> Manfred
>
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-- 
R. Kevin Stover
AC0H
ARRL
FISTS #11993
SKCC #215
NAQCC #3441


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