I have been following this thread with great interest. I have built two
SS linear amplifiers, one at a kilowatt and one at 600 watts. Neither
of these amplifiers are my designs. Both were designed by Helge
Granberg and promoted by Communications Concepts. The kw amp uses a
pair of MRF154 FETs and the 600 amp uses four MRF150 FETs. These
transistors are relatively expensive so I was reluctant to branch out
with my own design attempts. My design contributions have been in the
power supplies and the control and protection circuits. They can be
seen in detail at http://www.tomthompson.com . Recently, however,
Freescale has released several LDMOS transistors that work well in the
HF range. The 300 watt part is $75 from Mouser. These transistors come
in a package of two which is ideal for a push pull amp. They are also
very rugged and thus ideal for amateur experimentation. I have, also,
recently completed a 100 watt class E amplifier drain modulated by a
Class D pulse width modulator for 160 meter AM. The Class E amp runs
93% efficient and the class D modulator runs 90% efficient. Both the
amplifier and the modulator use MOSFETs that cost about $2. With these
kinds of efficiencies, the power supply is easier, too. I would
encourage some of you to experiment along these lines. The price and
ease of SS experimentation is within the reach of many of you.
73, Tom Thompson W0IVJ
On 5/2/2012 8:34 PM, Manfred Mornhinweg wrote:
> Roger, and all,
>
>> IF and I have to emphasize the IF these transistors were rugged enough
>> for prime time and I'm quite willing to take your word on the power you
>> are getting out, the manufacturers would be jumping on them like flys
>> on...er... honey because they could run them PP/parallel for the legal
>> limit out plus comfortable orverhead and couldn't build them fast enough
>> to meet the demand. There has to be a reason they are not doing so.
> In fact I have been wondering about this question. The reasons I can see
> for commercial manufacturers not doing it this way might be both
> technical and commercial:
>
> - Hams are obviously willing to pay 5000 bucks for an amp, so why
> destroy the market by making amps that sell for 700 bucks? The
> intermediate option, making amps that are cheap to make, and then sell
> them at 4000 bucks, probably won't live long with hams. They want to see
> where the cost is.
>
> - There are several 30 to 40 year old designs with 50V FETs that
> actually work, sort of. So let's just use these, instead of investing
> money in developing a new circuit around new FETs. There you have all
> those MRF150 amps. And the old designs are copied COMPLETE, even with
> the obvious bugs many of them have!
>
> - New designs = new problems, so... STAY AWAY! The present amps are
> still selling well enough, so don't touch them!
>
> A real technical difficulty with the type of amplifier I'm working on is
> that it really only gets good with water cooling. Trying to use air
> cooling, more individual devices are required, and the additional
> capacitances quickly stop the show, specially when combined with the
> additional wiring.
>
> And another technical problem is that my amplifier probably won't end up
> particularly rugged. If you set your mind on it, you can probably blow
> it up just by slowly tuning an antenna tuner through its whole range,
> until you find a setting with sky-high SWR that just happens to create
> high enough drain voltage peaks to fry the FETs. Protection circuits can
> keep FETs alive, as long as the high SWR comes up slowly (milliseconds),
> but if you have a loose connection in the antenna system, that provides
> 1:1 SWR most of the time, but suddenly goes up to a condition that
> causes those high voltage peaks, this amplifier can be blown up more
> easily than one that uses just a few, rugged FETs. The reason is that
> I'm using many small FETs, directly in parallel. That's simple to do,
> and inexpensive, but any overvoltage will make a single FET enter
> breakdown, and that single FET cannot survive the big discharge. When
> using just a few large FETs, which is much more expensive, the big FET
> that breaks down has a much larger chance to survive.
>
> For this reason, an amp like I'm developing is more suitable for the
> homebrewer than for the appliance operator. FETs will blow just under
> totally abnormal conditions, such as arcing over a loose antenna
> connection. With some care, such a situation can be largely avoided. And
> IF it happens, the FETs are inexpensive, and easy to replace. But an
> appliance operator is more likely to have such poor connections in his
> antenna system, and if he blows a FET, he has to send back the amp to
> the manufacturer, have a professional spend time on it, and this sort of
> masks the cost difference between a 1.5 dollar FET and a 150 dollar FET.
>
> So far I have killed no FET at all during all of my experimention with
> this amp project! But I'm far from done. I probably will kill some,
> while testing to the limit at high SWR. Anyway, they seem incredibly
> forgiving of mistakes. When semiconductors started, over half a century
> ago, oldtimers successfully made many people believe that those
> newfangled tiny things were terribly fragile, and this reputation has
> stayed. In truth, they are far more robust than most people think. Most
> FETs can take a high voltage spike that makes them break down (note to
> newcomers: "breaking down" in this context means "abnormally and
> unintentionally entering conduction", and not "failing"), and suffer no
> damage at all, as long as a certain energy per event isn't exceeded, and
> of course as long as the device isn't overheated through repeated
> breakdown or other causes. And regarding static charges killing MOSFETs,
> in all my 30 years working in electronics I have killed only one single
> component that way! And that was a "joint venture" with a colleague: I
> had just successfully repaired an ultra sensitive front end card that we
> urgently needed for the place's scientific work, and I triumphantly
> waved it at my workmates. One of them came over, walking across the
> carpet, without any antistatic gear (we didn't use any, back then), and
> took the card from me. ZAP! We both saw the spark from his hand to the
> card's edge, and we both felt the jolt. And then I had to repair the
> card again.
> But that event was highly exceptional. Most semiconductor parts,
> including all power FETs I have ever used, have built-in protection in
> the form of zener diodes, that keep the devices alive despite the usual
> small sparks that happen in daily life. It takes an exceptionally large
> discharge to kill a part. Some specific parts, such as certain ultra low
> noise microwave FETs, and many laser diodes, are unprotected against
> static. They need to be handled carefully, but it's nothing out of this
> world.
>
> On the other hand, I remember a terrible event in my other hobby, that
> of restoring antique radios. I needed a good-enough 2A7 tube for the
> front end of a beautiful 1933 cathedral radio. So I took my big box of
> old American tubes out of the cabinet, and went through it looking for a
> 2A7. I found two. Then I turned to my tube tester to see which was
> better, and something, somehow, I don't know how, got tangled, and the
> whole box of tubes fell to the floor! More than half of the tubes were
> broken, around 30. Talk about fragility! Semiconductors at least don't
> have this particular sort of fragility.
>
>> As to home brew, there are very few of us who actually do that be it SS
>> or tubes.
> I can't help it, I worry that this is spelling the end of ham radio. To
> me, ham radio is about being interested in radio technology. I can
> perfectly well accept the fact that not all hams are professionals in
> electronics, but I would expect that all hams, without exception, should
> at least be interested in electronics, and try to learn, at their own
> rate. I just can't understand those people who say they love ham radio,
> but whose love for radio ends precisely at the front panels of their
> rigs! To me, they DON'T really love radio. Instead, they are after the
> utilitarian side of ham radio, such as keeping in touch with friends and
> family, or having emergency communications. In my surroundings, at least
> 90% of hams, and probably much more than that, belong to this group that
> doesn't have any interest whatsoever in electronics. Hear them talk on
> the bands: The weather, the brand and model of their rig and antenna,
> and that's about it. I call them "empty QSOs".
>
>> I'd be interested in the device, and circuit design.
> What I'm doing is loosely based on DL9AH's designs. Google for his call,
> and you will find lots of information, most of it in German, but at
> least you should understand the schematics and photos... But I'm using
> adequate cooling, which he didn't, and also I'm using a much more
> decent, even if still very simple and inexpensive power supply. The
> intention is to package all that with a set of auto-bandswitched low
> pass filters, and sufficient protection to keep the FETs alive in all
> normal situations. I have tried several newer FETs than the ones Arno
> used, obtaining better performance, but I don't want to give away all
> details of what I'm doing, until I'm through with it and get it published!
> Anyway, if these messages spike some of you to start working in the same
> line, I would consider that to be very good, and would love to exchange
> experiences.
>
> Take DL9AH's published design, put in a larger number of newer and
> better FETs, add water cooling, replace his half-wave power supply by a
> switching supply with improved power factor, replace his somewhat
> problematic RF transformers by ones that work well, add autoswitched low
> pass filters, and there you have what I'm doing!
>
> I'm not nearly ready, in any case. I'm rather in the early stages!
>
>> At any rate all commercial amps and projects I've seen had very
>> elaborate protective circuitry which I said before was pretty simple
>> when looking at each section, but kinda intimidating when looked at
>> overall. Course most people make the mistake of looking at the whole
>> thing instead of one step at a time.
> People who have never built any equipment of significant complexity can
> of course get scared by such a circuit, that might have 70 parts or so.
> But it falls into proportion when you realize that these 70 parts cost 2
> 20 dollars total, and fit in three square inches of printed circuit space.
>
> Tell these people to look into their HF transceiver, and count the
> parts, and try to understand it all. That's quite a bit more complex.
> But even the most advanced HF transceiver consists of nothing but very
> plain, simple individual circuits.
>
> Of course, anything like that is too complex for the "typical ham", the
> one at whom the ARRL is addressing QST. But even the simplest tube type
> amp is neither for the typical ham, because that typical ham would never
> dare to build anything having 3000 volt in it. Or rather, he would never
> attempt to build anything at all, period!
>
>> I still maintain SS is not ready for prime time, but for those with the
>> knowledge and desire to experiment it's a great field.
> Well, I think differently: I think solid state is the way to go,
> definitely, at the power levels hams are allowed to use. Only at much
> higher power levels do tubes make real sense. Simply because tubes are
> available for several tens of kilowatts in a single device, while
> transistors are not, and would need to be stacked up in too large
> quantities. But at 1.5kW, and even 10kW and some more, solid state is
> more practical than dealing with tubes and their inevitable
> narrow-banded tuned circuits, their lifetime of a few thousand hours,
> and sockets costing a thousand dollars!
>
> One could argue that a lifespan of a few thousand hours for a tube will
> last a ham's lifetime of normal operation. But practical experience
> shows that tubes are less reliable: Among the local hams I know and who
> own power amps, the spread is about 2/3 tube amps, and 1/3 solid state,
> changing fast to increase the proportion of SS. Most of these hams turn
> to me when something fails. Over the last several years I had to replace
> several 3-500Z, and a few other tubes, but not a single high power
> transistor. The only time I got a solid state amp for repair, a Quadra
> that had seen extensive contest and DXpedition use, the problem was a
> broken solder joint that took five minutes to find, and one second to
> repair.
> In several tube amps I had to replace electrolytic caps, and I had to
> rewind two power transformers. In SS amps, so far I have seen no power
> supply trouble. Of course, the number of amps that could possibly fall
> in my hands when broken is small, but I believe that it's enough to do
> at least a broad statistic about their reliability.
>
> Years ago I looked into the possibility of making broadband tube
> amplifiers, because I just hate having to retune an amp after changing
> bands or even changing frequency a lot within a band. But I quickly
> found that the tubes' combination of high internal capacitance and
> required high operating voltage makes this totally impossible. Tubes
> force the use of tuned circuits, while transistors give the designer the
> choice between tuned and broadband impedance matching.
>
>> It's also, often at pulse with enough off time to allow the heat to
>> migrate to at least the spreader
> Indeed the datasheets of many RF transistors rate the RF output power
> under pulsed condition, and I have seen hopeful hams who thought they
> could run that power continually...
>
> In pulsed service it's quite complicate to correctly calculate the
> thermal limits, because the distribution of thermal mass and thermal
> resistance within the device has to be considered in a detailed way. So
> it's good that manufacturers specify the ratings for pulsed use. But it
> would be really nice, and in the best interest of honesty and
> transparency, if they would also rate the actual power output under
> continuous two-tone conditions in class AB, while using real-world heat
> sinks! But then, many of these transistors are simply not intended for
> us hams, nor for any other communications use. They are intended for
> nuclear magnetic resonance imagers and other such big money devices,
> which indeed work in pulsed mode.
>
> The power dissipation ratings in the data sheets are instead in steady
> state conditions. At least I haven't seen anything other. And this is
> what brings about data sheets of transistors that tell that you can get
> 1.5 kilowatt RF output, while the device has an absolute maximum
> dissipation rating of 600 watt. The output rating is in pulsed service,
> while the 600 watt dissipation is continuous, but assuming a case
> temperature of 25 degrees Celsius, which can hardly ever be achieved in
> practice (a beacon transmitter at the south pole might be the
> exception). To a ham, these ratings mean that you can run perhaps
> 300-400 watt output in SSB, class AB, or about 600 watt output in CW or
> FM, running class C. But certainly not 1.5kW, unless you go to class E
> or class F - which is actually a very good way to make good use of these
> transistors!
>
> Tube dissipation ratings instead are true, real, directly usable, as
> long as you assure enough airflow. This difference comes from the simple
> fact that tubes usually have their heatsinks built-in, while transistors
> do not, so transistor ratings have to be given in a way that allows
> equipment designers to calculate the true allowable dissipation, after
> fitting a certain heatsink.
>
> Note that some solid state devices, such as many diodes and also some
> switching MOSFETs, nowadays are rated for dissipation and/or current at
> a more real-world temperature, such as 75 or even 100 degrees Celsius.
>
>> Depending on frequency we set the internal limit at 100C and max
>> internal operating temp at somewhere between 70 and 90C.
>> dopant migration was starting to become noticeable at 100C so when I was
>> in the industry.
> That's a bit lower than I had known. Anyway one has to keep in mind that
> these phenomena are time-related. A ham amplifier can probably get away
> with pushing the silicon to 150 degrees when delivering full output,
> because this will only be short-term. If the transistors live for 10,000
> hours at that temperature, that should be enough. A broadcaster instead,
> running 24/7 at full power, will want to get much more than 10,000 hours
> of life from his transistors, and so they need to run cooler.
>
> That said, and having done calculations on the thermal aspects of
> widespread ham equipment, I can't avoid the suspicion that many ham
> transceivers run their finals well in excess of 150 degrees internally!
> The typical max ratings are often 175 degrees or even 200 degrees for
> transistors that are NOT encased in plastic. Particularly brave
> manufacturers even specify 225 degrees. I don't know how much of that is
> due to processing techniques that reduce dopant migration. The rest is
> probably based on decisions to accept shorter lifespans as "typical". In
> any case, silicon by itself survives much higher temperatures. It's the
> migration problem that sets a limit, and the attachment of the chip to
> the case (usually by some solder) sets another limit.
>
>> You can also use *cold* or chilled water. Just draw the water out of an
>> open PVC tank and return to the same tank. 20 gallons will run a KW amp
>> for quite a while. Throw in a few chunks of ice and have at it. We use
>> the water in plastic bags for our picnic coolers , so you just grab a
>> couple and drop in the tank
> But one needs to be always careful about avoiding condensation inside
> the amp. At my former job, we had tons of actively cooled electronic
> equipment. There were big coolers to chill the water/glycol mixture just
> enough to keep the outside of the electronic racks precisely at ambient
> temperature, to prevent thermal turbulence. The controllers monitored
> ambient moisture, to keep the water always above the dew point.
> Sometimes something went wrong, and then we got dripping wet
> electronics, with all the obvious consequences.
>
> The coolers were mostly of the plain common compressor heat pump type,
> but in some vibration-sensitive locations we decided to try Peltier
> coolers. They turned out to be a disaster. The place was at 2600 meter
> altitude, the Peltiers overheated due to the low air density, and died
> faster than we could throw them in the trash.
>
>> You can take it down as far as you like as long as you don't get
>> condensation on the circuit board,
> Exactly. And with a typical comfortable ambient humidity of 60 to 70%,
> condensation happens at just about 6 degrees below room temperature!
> It's not worth doing, for such little temperature difference.
>
>> or ice up the pump.
> At any normal room temperature and humidity, condensation will occur far
> sooner than any ice can form. The dewpoint at comfortable room
> conditions is always above 10 degrees Celsius. You would need an
> extremely dry or very cold environment, to get the dewpoint below freezing!
>
>> BTW there is a fungicide used in swimming pools that will do a good job
>> of keeping *stuff* from growing in your cooling system. I'm not
>> referring to Chlorine.
> I will have a look in the swimming pool section of the home improvement
> store! Thanks for the hint!
>
> Manfred
>
> ========================
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> http://ludens.cl
> ========================
>
>
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