Op Amps for Wenzel Circulator - Page 1
Forum section: electronics / rf-microwave

author: uofmrob
author_rank: Regular Contributor
timestamp: October 05, 2020, 11:23:20 pm
content: Hi everyone,I had a thread about this on a separate part of the form and all the great help led to an awesome working Wenzel circulator. I used the attached design with the quad op amp IC of the LMH6722. However, it cannot handle the signal voltage and current that I need, so I'm trying to make a second one. I bought a quad op amp of a Linear Technology LT6301. I made it the same way and double and triple checked everything and it's not working properly. Is there a difference between a typical op amp and a "differential xDSL line driver" as the LT6301 is described? I added the control voltage and resistor to ground on pins 4,5, and10,11 as described to set the current in the documentation. It didn't seem to work. In the working one, I get a 1:1 transfer of the signal to the next port and it carries forward to the next port if reflected, but with this second one, I'm only getting about 20% carrying forward.Any help would be greatly appreciated!Rob

author: coppercone2
author_rank: Super Contributor
timestamp: October 06, 2020, 06:24:31 pm
content: that circuit uses a gain of 3.26, the amplifier you have has a minimum stable gain without compensation of 10, perhaps thats something to do with it? I don't think the wenzel circuit is designed to compensate your op amp, if thats the case.See if you violate any page 11 guidelinesan appealing choice of part, if you can get it to work

author: uofmrob
author_rank: Regular Contributor
timestamp: October 06, 2020, 06:46:58 pm
content: Thanks for the input, coppercone2. I'm a chemist by trade and just kinda delving into this stuff, so I'm certainly flying by the seat of my pants.I was mostly just looking at the power and voltage handling as I'm certainly not an expert at these things. So it's not surprising that I overlooked something. I have another part in hand that might work.https://www.analog.com/media/en/technical-documentation/data-sheets/AD8392A.pdfDo you happen to see any red flags with this one?Thanks!Rob

author: uofmrob
author_rank: Regular Contributor
timestamp: October 06, 2020, 07:17:35 pm
content: Perhaps this might be another option?https://www.renesas.com/us/en/www/doc/datasheet/isl55004.pdfWith the one that I made, the maximum input voltage is 2.2 V. Setting that on my signal source, I'm getting about 8 mW of power. I'm not seeing anything on my spectrometer that I'm making at that power. I would like to get it at least to 50 mW, but 100 mW would be great, but I don't want to fry the op amps.

author: uofmrob
author_rank: Regular Contributor
timestamp: October 06, 2020, 07:20:55 pm
content: I should also mention that I only need it to perform well up to maybe 40 MHz or so.

author: David Hess
author_rank: Super Contributor
timestamp: October 06, 2020, 08:01:47 pm
content: If coppercone2 had not said it I would have; the LT6301 needs to be compensated to operate it at a lower gain as described on page 11 of the LT6301 datasheet.

author: uofmrob
author_rank: Regular Contributor
timestamp: October 06, 2020, 08:17:57 pm
content: Thanks! So looking at this, am I thinking correctly in that something like a 50 ohm resistor from the negative input to the ground would work for compensation?

author: coppercone2
author_rank: Super Contributor
timestamp: October 06, 2020, 08:18:22 pm
content: If you do it, just make a standard amplifier (like a gain stage), and make that work first, before you tackle the entire feedback loop. So the first step would be to make it work with G = 3. If you don't know a single stage works, you might have something like a damaged part thats going to drive you crazy (being new) if you try it on 3 amplifiers at once. Verify it piece by pieceMy project on this uses a CFB amplifiers, and I don't want to divert my attention to a modified circuit right now. If you have a amplifier of the same type they show in that circuit, so long you don't see anything related to stability clauses, you are ok

author: uofmrob
author_rank: Regular Contributor
timestamp: October 06, 2020, 08:27:28 pm
content: You might be able to find a ferrite based circulator cheap on eBay. I've seen them on occasion for $40-$50.

author: coppercone2
author_rank: Super Contributor
timestamp: October 06, 2020, 08:29:34 pm
content: I think I have one, or at least have isolators, but the circuit is interesting to me because it is active (always had a interest).

author: uofmrob
author_rank: Regular Contributor
timestamp: October 06, 2020, 08:30:32 pm
content: The only reason I'm trying to figure this out not just use a ferrite circulator for this is the low frequency.

author: coppercone2
author_rank: Super Contributor
timestamp: October 06, 2020, 08:32:12 pm
content: the part you chose is unity gain stable, but keep in mind the specifications are for a 1kOhm load. If you want to drive 50 ohm load it might have trouble.The first part you have is 500mA. The 2nd part isContinuous Output Current . . . . . . . . . . . . . . . . . . . . . . . . . . . 60mA maximumSO its alot weaker. Make sure its enough for what you need. Making this circuit high power is not trivial I think. I think based on your needs for power, you might want to mess around with the compensation scheme. (good project, actually considered advanced to compensate an op amp)The DSL part is a good choice because its mass produced for DSL, so its cheap, but keep in mind, many people are stopping providing DSL, so it might not be forever. Granted by now I assume many other applications were developed for a capable amplifier like that.

author: uofmrob
author_rank: Regular Contributor
timestamp: October 06, 2020, 08:40:39 pm
content: It's part of a home built EPR spectrometer that I'm building. The first circulator worked perfectly, but I wired everything up, put the sample in and nothing. I checked the magnetic field and it was dead on with the calculations, brought the test sample (coal) sample in to work, and looked at it on a research instrument and the signal was as big as a house. The saddle coil that I made resonates at 25 MHz beautifully with the tuning circuit and testing with a little pickup coil showed that the RF B1 is perpendicular to the magnetic field as it should be. The problem seems to be not having enough power. So I have a little voltage controlled RF amp that goes from -2.5 dB to 42.5 dB amplification. But I need a circulator that can handle the power. If a ferrite one was possible at this low frequency, that would be perfect.

author: uofmrob
author_rank: Regular Contributor
timestamp: October 06, 2020, 08:43:54 pm
content: The moment of disappointment.

author: uofmrob
author_rank: Regular Contributor
timestamp: October 06, 2020, 09:02:29 pm
content: And just for reference, this is the working low power circulator made from the LMH6722 quad amp IC and a lot of help from this crowd over on the EEVBlog suggestions channel. I had posted that the Wenzel Circulator would make a cool EEVBlog video and it ended up being a long thread with a lot of suggestions that netted this first working one.

author: David Hess
author_rank: Super Contributor
timestamp: October 06, 2020, 09:32:19 pm
content: Quote from: coppercone2 on October 06, 2020, 08:32:12 pmI would consider using dual or quad amplifiers in parallel for greater output capability, but this can be very tricky.

author: coppercone2
author_rank: Super Contributor
timestamp: October 06, 2020, 10:02:24 pm
content: im interested in that box, do you make those? is that metal?uncanny because I built something almost identical out of wood just now (mitered edges)

author: uofmrob
author_rank: Regular Contributor
timestamp: October 06, 2020, 10:30:35 pm
content: No, is a crappy little custom designed 3D printed box.i can give you the STL file if you want. It worked out well, but it's nothing fancy. It fits the 100 mm x 72 mm copper PCB boards that I bought.

author: coppercone2
author_rank: Super Contributor
timestamp: October 06, 2020, 11:06:01 pm
content: nah for some reason I thought you mitered aluminum plate and screwed it together and I was doing it with wood and it did not come out that great and I was thinking if it would be hard to do it with aluminum with basic tools (always looking for a high end noob box)As for first working one, I think I posted a thread about this a year or two ago and people claimed to have built them, but I am not sure if anything with 'non stock' parts was announced publicly yet. Defiantly the first usable one I saw that would make sense on a work bench. I just don't like that it is not battery powered.

author: uofmrob
author_rank: Regular Contributor
timestamp: October 07, 2020, 05:03:42 pm
content: What does everyone think about trying to get a hold of 4 of these?https://youtu.be/mEFdADrU9MA

author: David Hess
author_rank: Super Contributor
timestamp: October 08, 2020, 01:32:40 am
content: The K2-W schematic is available if you want to make one or several.

author: uofmrob
author_rank: Regular Contributor
timestamp: October 08, 2020, 02:06:45 am
content: Yeah, I was making a joke. The thought of trying to make it stinging together 4 of these with 300 V supplies with be hilarious

author: coppercone2
author_rank: Super Contributor
timestamp: October 08, 2020, 02:12:30 am
content: ruggedized power hungry CB radio?I don't like any application of this thing because its so power hungry and hot.im really not sure if there is an appeal of using a transmit antenna for a receive antenna for a ham in a electrically efficient sense with all that power being used to heat the tubes. I think they would have a electrically small receiver and a transmitter in most cases. It's neater and more reliable I think, then linking two systems on the signal path directly. sounds like civil defense stuff (too heavy for the modern military?)I would try to find a good use for it before building it, sounds kind of like deciding to armor plate your bath tub.

author: coppercone2
author_rank: Super Contributor
timestamp: October 08, 2020, 05:10:14 pm
content: I like the idea of a vacuum tube buffer output for a power supply, or vacuum tube output stage, because I have fried PSU with stupid things that can be prevented, but for a radio, I feel like its very unlikely to break it by testing a new antenna, and its not worth running the circuitry because its inefficientMaybe a protection circuit you turn on for trouble shooting suspicious amplifiers that can present a bad load?i want to make one myself , if I can find a use, because it is cool, but I doubt anyone will come up with a purpose??It's not as outlandish as you think, Jim Williams did include a vacuum tube output stage for a power supply in one of his app notes, I think it was semi-serious.. I think it would be useful to have that power supply for dealing with solenoids. I actually thought about designing a PSU like that, just so I could maybe connect electromechancial stuff without trying to figure out snubbers ETC, with higher confidence, for serious electromechanical research involving valve timing, that benefits from a stabilized power supply (for repair purposes, a battery bank is WAY more suited for this), but if you are interested in precise timing etc, you could benefit from a stabilized rugged tube supply, to increase ease of use.Is there some nasty HV RF thing that destroys sources and could benefit from a circulator? Tesla coils come to mind.. (if there was a useful industrial process behind tesla coil use, for me to justify expenses at least)

author: coppercone2
author_rank: Super Contributor
timestamp: October 10, 2020, 01:27:57 am
content: is the K2-w the most advanced produced op amp with vacuum tubes, or are there better versions?the K2-w works to 300 kcycles, is there a faster standard circuit?https://www.electronicdesign.com/technologies/analog/article/21768932/whats-all-this-k2w-stuff-anyhowlight readingdoes anyone have an idea for a faster standard vacuum tube op amp circuit to use with a active circulator?

author: David Hess
author_rank: Super Contributor
timestamp: October 11, 2020, 12:23:36 am
content: Quote from: coppercone2 on October 10, 2020, 01:27:57 amIt was relatively cheap with sufficient performance to be useful.

author: coppercone2
author_rank: Super Contributor
timestamp: October 11, 2020, 12:26:05 am
content: do you know of a more advanced circuit that could be used for a higher speed vacuum tube circulator?

author: David Hess
author_rank: Super Contributor
timestamp: October 11, 2020, 12:51:29 am
content: Quote from: coppercone2 on October 11, 2020, 12:26:05 amI do not know offhand, but typically a faster design would be done from scratch. The K2-W was designed for economy.The usual rules for operational amplifier design apply; reduce the transconductance of the differential stage so that the tail current can be increased the the compensation capacitance decreased. Add a couple resistors to degenerate the K2-W differential pair and reduce the compensation capacitance.A current feedback (cathode feedback) topology would be faster. Find some tubes made of antimatter and you can build a complementary current feedback amplifier.It is fun to consider what all NPN, n-channel, or tube topology would be fastest.

author: coppercone2
author_rank: Super Contributor
timestamp: October 11, 2020, 01:22:43 am
content: what fast tube would be a good place to design around? not that in intend to do this any time soon, if at all, but its interesting, and financially perilousI assume it would be possible to do it with things like TWT's also, but thinking about that makes me wanna vomit almost

author: David Hess
author_rank: Super Contributor
timestamp: October 12, 2020, 02:09:26 am
content: Quote from: coppercone2 on October 11, 2020, 01:22:43 amI would experiment with a small signal RF pentode, maybe the 6AU6.

author: coppercone2
author_rank: Super Contributor
timestamp: October 12, 2020, 06:45:05 am
content: mm pretty cheap