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Author Topic: Anyone have experience building "Envelope Elimination and Restoration" Amp?  (Read 265 times)

KG5KCW

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I am looking at an October 2010 QST article titled "Homebrew Challenge II Co-Winner -- The Lowest Cost Entry"  David Cripe, NM0S.  It describes a 40 meter 50W Class E linear amplifier using two IRF520 FETs.  The FETs are driven as switches with SSB phase preserved, and the SSB envelope is added by modulating the FET drain feed voltage.  The technique is called "Envelope Elimination and Restoration".   This seems like a nice scheme when using cheap power FETs, since it avoids Vth drift induced stability and thermal runaway problems associated with operating power FETs as linear amps.

Has anyone out there tried this technique? Does it preserve sideband suppression well?

--KG5KCW, Steve

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VK6HP

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Hi Steve,

Thanks for posting the NM0S reference - I hadn't seen that, and would suggest that it represents just about the most minimal EER implementation. 

I can't claim to be any authority on practical EER but I did try it for fun on the bench with my 300 W Class D/E PA for 475 kHz, as well as a 175 W single-FET Class E PA for the same band. All of my current LF/MF amplifiers use IRF250 FETs, although I plan to switch to newer SiC devices for the next generation of PA's.  I strongly advise against using the power FET's as linear devices: it can be done (as you no doubt know) but the relatively poor linearity and thermal behaviour is more trouble than it's worth in my view; if you want a conventional RF amplifier, buy RF FET's is the lesson I've learned. (I haven't blown up a FET since moving to Class D/E!).

My lash-up worked pretty well and I had no trouble recovering good SSB originating in a TS-590S exciter (using the DRV output).  I'm quite pedantic about good input squaring to the Class E section, and I use an LT1016E precision comparator to achieve that.  Good stiff gate drive using e.g. one of the many driver chips is also well worth it.  At the output, it's worth spending time aligning the time response of the FET(s) and the output network, in terms of harmonizing the forced and natural responses as described by Sokal et al.  Even in FSK input modes I've noted the near-in distortion of the amplifier in terms of unwanted mains-related and other products is a function of load-pull and, while I didn't do any measurements in that area on the bench, I might speculate that a good output match would help with clean SSB.

I hope that you get some more responses from people who've tried the ultra-simple QST concept and who can comment on how it worked out.

73, Peter.
« Last Edit: March 23, 2023, 02:28:15 AM by VK6HP »
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KG5KCW

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Hi Peter,

Thanks very much for your detailed reply.  It's very helpful.  I've decided to go ahead and build up one of the NNM0S EER amps.  It shouldn't be that difficult or expensive. I have an old Ramsey QA 20W amp that I might try to modify for EER too.

I was browsing the Nov. 1978 issue of Ham Radio and came across an article "Mosfet Power Amplifier for Operation from 160-6 meters".  It was basically an early introduction to using power FETs as RF power amplifiers. It discussed FET PAs as strictly Class D/E and mentioned the EER technique as the way to amplify SSB signals. Searching around using DuckDuckGo.com eventually led me to the NM0S EER article.   

Before retiring I used to design high temperature circuits for oil well logging tools. I also did quite a bit of work on high temp electronics back at the university.   Mostly Silicon on Insulator circuits and SiC transistors. The SiC market was just getting started back then and there wasn't a clear indication of whether SiC JFETs or MOSFETs would become dominant.  I haven't looked at the SiC power FET market in a long time, so I don't know who the manufacturers are or what are the costs. It always seemed to me that SiC weren't worth the high cost when the ambient temperature was only 25C.   

I am not familiar with the Sokal reference, is it a book?  I found the Gregennikov and Sokal book "Switchmode RF Power Amplifiers",  which looks very promising.  I'll have to go to a local university library to check it out, since I can't find a cheap used version on abebooks.com   

I would also like to contact you and see if you've got any schematics or notes to the EER mods you did to the 300W and 175W MF amplifiers that you mentioned.

Thanks kindly,
Steve, KG5KCW
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VK6HP

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Hi Steve,

Nathan Sokal (WA1HQC) (SK) patented the Class E amplifier concept and, in addition to his IEEE and other professional papers, wrote quite extensively in the ham literature, especially QEX.  In particular, his Jan. 2001 QEX paper, and follow ups over the next few months, is an excellent and very practical design overview.  Interestingly in the present context of EER, his earlier QST letter of Jan. 1998 reports on some experiments with EER in which he was able to obtain < -40dBc IMD for an 80 m power amplifier.

The book to which you refer is an updated version following Sokal's death in 2016 at the age of 87.  The current version is "Switchmode RF and Microwave Power Amplifiers" by Grebennikov and Franco, Academic Press, 2021. I borrowed a copy via an inter-library loan before eventually deciding to buy a paperback edition. After the engineering component of my career was spent worrying about microwave and millimetre-wave ultra-low-noise receivers, it's a contrast to be spending some of my declining years building LF/MF transmitters!

My own bench experiments were very informal and undocumented, filling a slow couple of days a few years ago when I had some ex-beacon PWM modulators in the shack, along with some hastily constructed Schottky envelope detectors. However, I do have details of the 175 W Class E PA and high speed input comparator if that's useful.  The 300 W Class D power amplifiers are are similar to those designed by David, G0MRF, although I drive the transformer-coupled push-pull arrangement with my comparator, eliminating the need for a double-frequency drive signal (which is normally divided by 2 to obtain the accurate gate drive square wave).

Regarding the SiC FET's, David and I have a new prototyping board incorporating selectable input type (flip-flop or comparator), gate dead-time generator using digital delays, FET drivers and FET mountings.  See David's picture at: https://twitter.com/g0mrf/status/1482828715545579531 to get the general idea.  It's not that we're planning to run the FET's hotter in operation; it's simply a way of increasing the ruggedness of the PA in response to extreme events (like the antenna falling down!).  Our amplifiers are well-protected but experience has shown that you can't have too much device robustness.  The cost of SiC devices is falling and they have a little advantage in terms of output capacitance if I recall correctly, so they begin to look reasonable for new designs. The devices I'm using are UnitedSiC UF3C065040K3S, by the way.

73, Peter.

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KD6VXI

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Cree Silicon Carbide FETs have been working into 40 meters OK.  They do take some good gate drivers.

Myself I've worked Class E RF decks up to the 10 meter range.  Built a PWM 500 watt PEP deck.  Was a replacement for a Johnson Viking in my mind, 100 watt carrier, 125 percent positive peaks.

Hermes with EER output is fun :)

--Shane
WP2ASS / ex KD6VXI
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KG5KCW

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KD6VXI,

That sounds really cool...  What's the part numbers of the SiC transistors you used?  Cree appears to have changed their name to Wolfspeed.   Looking at Digikey, it looks like some SiC device prices are lower that $10.  Do you have notes on the 500W PWM deck on a blog or website somewhere?  I'm very interested to see how you implemented the SiC drive and the envelope modulation...
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W1RKW

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anyone have a working schematic of this type of amplifier?
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