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Author Topic: Why no class E or D, HF amplifiers?  (Read 3292 times)
W8JJI
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« on: January 04, 2008, 10:01:49 AM »

With these highly efficient transmitters available to hams, often only pulling 6 amps or so from a standard 110v household AC line and making a 300 watt carrier with little heat dissapation, why are there no HF amplifiers available that use class D or E desighn ?
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KB9VLR
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« Reply #1 on: January 04, 2008, 10:35:44 AM »

Class D and class E amplifiers are "switching" type of amplifiers. Signals from these amps would have a lot of harmonics that wouldn't play nice on the HF band. This would likely lead to some upset neighbors as well. The choice of amplifier design is also based on the mode of operation (SSB, AM, FM, etc), and the bandwidth the amplifier will operate over. Some modes tolerate distortion in the amplification better than others.

Class A, B and AB amplifiers are linear in nature.  (hence the name "linear") Linear means that the characteristic shape of the waveform entering the amp is preserved at the output. In order to do this, the transistors performing the amplification must be in the "ON" state at all times, which consumes a lot of extra power. The result is a much more pure "clean" output.

Class D and E amplifiers are more efficient because there are periods of time where the transistors are not conducting (in the off state). This distorts the waveform, and introduces clipping. Clipping and chopping parts of the waveform create the harmonics.

Take a look on Wikipedia, there is some decent info there on how amps operate.

Adam
KB9VLR
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WB2WIK
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« Reply #2 on: January 04, 2008, 10:35:49 AM »

These are switching amplifiers requiring switching at frequencies much higher than the signal frequency, so for a 30 MHz amplifier we may be talking about a 300 MHz switch.

Then, those pulses and all the harmonics generated therefrom must be filtered out before you could provide the amplified signal to an antenna.

Not economically feasible yet, although they're becoming more common for audio amplifiers where all this is much lower frequency and thus more easily accomplished.

WB2WIK/6
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NH7O
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« Reply #3 on: January 04, 2008, 10:39:37 AM »

They are out there:

http://www.classeradio.com/

The harmonics can be filtered well enough, so that is no problem. But Class E as shown above, does not allow for linear SSB amplification, only CW or AM.
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WA3SKN
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« Reply #4 on: January 04, 2008, 10:53:07 AM »

Well, there are!... sorta!
These switching devices are not practical for amateur use.

-Mike.
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N3OX
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« Reply #5 on: January 04, 2008, 12:45:14 PM »

Probably too much of a niche market to be profitable.

As has been pointed out, there are issues with linearity and possibly switching speed... and the Mosfets so useful for these things start to exhibit severe gate capacitance issues at high frequencies...

I wonder if it's just that amp manufacturers want to sell **HF amps**... that cover 1.8-30MHz and put out legal limit across that range and you just can't do that with class E amps...

I would probably buy a 1500W 1.8 & 3.5MHz  CW-only Class E amplifier if it meant it came in at a price point well under a legal limit all-HF band amp of some other kind... and I think maybe you could do that, switch mode power supply, inexpensive FETs, doesn't need much cooling, etc... but would it sell in general?  I dunno.


Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KZ1X
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« Reply #6 on: January 04, 2008, 01:26:42 PM »

Wasn't the SGC modular amp, the one that never got to market, a Class D amp?
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N3OX
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« Reply #7 on: January 04, 2008, 01:48:11 PM »

I think it was Class E

It's called the Mini Lini, came across it while searching for more info.  It used plug in modules to change bands and was only going to ship with a 20m one ,but they had issues with the connectors between the main box and the plug-ins.

Seemed foolish to not offer a choice of plug-ins to me anyway... I'd much rather buy the thing with 160m and 80m modules and no 20m.

That thing made sense for mobile/portable use... as do all high-efficiency amps, and that should drive some of these things to market.

A Mini Lini would have been a non-starter for home use though I think... efficiency just isn't the #1 thing people want and at $600 for a single band 500W amp, why not just spend $700 for something like an AL-811 and get 600W?

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
AC5UP
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« Reply #8 on: January 04, 2008, 04:22:52 PM »

Coincidently, I've been doing some recent web surfing on the concept of amplitude modulation and Class E. As I understand it the magic is in the pulse width modulator that switches the final(s) on and off rapidly enough to replicate an old-school plate modulated Class C final with better efficiency and linearity.

This is nothing new and is adaptable to both tube and solid state designs. Rockwell-Collins marketed a 'Power Rock' series of BCB transmitters in the 70's that were tube final hybrids as the competition was moving toward 100% solid state. They claimed a lower cost of ownership with less down time using tube finals, but that was then. If you want to see how not-new this is, here's a pic you'll find amusing:

http://www.jtml.info/314R1/PowerRock1-BrochureLg.jpg

Disco Stu, broadcast professional.... ( ! )

Visit the Class E Radio web site for more, and if you want to see what a typical commercial implementation looks like, here ya' go:

http://www.radiosystems.com/Manuals/tr-6000.pdf

Interesting read.

Could this be applied to amateur SSB use? Yeah, but that's a long road to get to where you already are and not likely to be cost effective. Besides, CW has been pulse width modulated since Day One. Wink

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KZ1X
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« Reply #9 on: January 04, 2008, 05:02:09 PM »

That brings back memories.  I installed the very first first Continental model 314R-1 Power Rock 1kW transmitter, at what was then WPCN 960 in Mt. Pocono, PA.  Had three tubes in it, one was the switcher and the other two, the PA.  I think they were 3-500z as opposed to the more-common in such gizmos 4-400.  4-tower phased array.

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AC5UP
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« Reply #10 on: January 04, 2008, 06:46:28 PM »

That means you have to visit this link... http://www.jtml.info/
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KZ1X
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« Reply #11 on: January 04, 2008, 07:26:33 PM »

Nelson

Wow, thanks!

This is especially nice treat for me, as my mentor/employer from those days is now an SK.

I have frequently thought about doing something much like this, getting one of the old Bessies I used to work on, fixing it up, and feeding it into my tower here, shunt-fed, for 160 AM.  

There's something that just seems right about that.

One day ...

73 Steve KZ1X/4
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WB2WIK
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« Reply #12 on: January 04, 2008, 09:13:03 PM »

>RE: Why no class E or D, HF amplifiers?  Reply  
by KZ1X on January 4, 2008  Mail this to a friend!  
That brings back memories. I installed the very first first Continental model 314R-1 Power Rock 1kW transmitter, at what was then WPCN 960 in Mt. Pocono, PA.<

::I didn't know you were doing that stuff, Steve!

I built the HV power supply for WMLK in Bethel, PA back in about 1978.  Not sure if they're still on the air or not, but this was a 1 million Watt SW BC transmitter with antenna aimed towards the Sinai Peninsula (religious station).  What fun that was!   The transmitter ran so much power, the Bird coupler for checking SWR had to be water cooled.  My part was building the HV PS which was 3-phase (of course) and delivered about 10kV at 50A, so each rectifier stack (per phase) was made up of about 20 rectifiers in series and 10 in parallel, with cooling fins between the cells and forced air blowing across them.

When the transmitter fired up for the first time, all the grass in front of the antenna (log-v, aimed into the ground for a bounce) burned up.

Fun stuff, eh?

WB2WIK/6
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KE3WD
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« Reply #13 on: January 04, 2008, 10:15:12 PM »

>>When the transmitter fired up for the first time, all the grass in front of the antenna (log-v, aimed into the ground for a bounce) burned up. <<


Don't let the enviroweenies hear stuff like that...


Oh, the poor earthworms.  


Reminiscent of the 450MHz jammers aimed at Cuba down in FLA, where the dead seagulls littered the beach.  


.
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AC5UP
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« Reply #14 on: January 05, 2008, 05:40:12 AM »

Back in the 30's Crosley broadcasting scored an experimental waiver from the 'C to run half a megawatt on 700 kHz @ WLW in Cincinnati. Here are a couple of web links to fill in the details...

http://www.oldradio.com/archives/stations/cinc/wlwpix.htm

http://hawkins.pair.com/wlw.shtml

Note that the transmitter was liquid-cooled via an outdoor pond. Made me wonder if the BC SW station mentioned previously ever considered a large-ish but shallow pond for the RF bounce pad that doubled as a heat sink. (?)
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