Hello all,
I have reached a point where I need to be able to analyse minor loops for stability. I have read about it and its just a mental block for me. I have been experimenting with bootstrapping and TMC. This now means I have nested minor loops within the main feedback loop.
Basic PM/GM analysis of the complete circuit reveals that it is stable on this level. However, I should be able to do the same with minor loops but just can't get my head round it 🙁
Please could someone give an idiots guide as to how this is done in LTspice and how to interpret the results. The examples I seen just look like hieroglyphics to me.
Any help would be much appreciated.
Paul
I have reached a point where I need to be able to analyse minor loops for stability. I have read about it and its just a mental block for me. I have been experimenting with bootstrapping and TMC. This now means I have nested minor loops within the main feedback loop.
Basic PM/GM analysis of the complete circuit reveals that it is stable on this level. However, I should be able to do the same with minor loops but just can't get my head round it 🙁
Please could someone give an idiots guide as to how this is done in LTspice and how to interpret the results. The examples I seen just look like hieroglyphics to me.
Any help would be much appreciated.
Paul
https://bspace.berkeley.edu/access/...454a58b9362d/Lectures/T11 feedback_simone.pdf
is good but doesn't really offer much hope - looks like dummies really need a few courses in (each of) Linear Algebra, State Space Control Theory
I only have the 1st courses of those - and a long career in Scientific/Industrial Instrumentation electronics design, some motion control - I never used State Space Math, analysis tools
http://web.mit.edu/klund/www/papers/ACC04_opcomp.pdf is about as hard as I ever needed
is good but doesn't really offer much hope - looks like dummies really need a few courses in (each of) Linear Algebra, State Space Control Theory
I only have the 1st courses of those - and a long career in Scientific/Industrial Instrumentation electronics design, some motion control - I never used State Space Math, analysis tools
http://web.mit.edu/klund/www/papers/ACC04_opcomp.pdf is about as hard as I ever needed
Thank you jcx for the links. The second one looks good... Shall print them out at work tomorrow.
Damn, I used to be able to do all this s domain stuff. That was back in 98. Now I have an interest that requires relearning (and actually understanding the underlying principles) all this stuff.
Its mad you end up studying harder for a hobby than for a qualification.
Damn, I used to be able to do all this s domain stuff. That was back in 98. Now I have an interest that requires relearning (and actually understanding the underlying principles) all this stuff.
Its mad you end up studying harder for a hobby than for a qualification.
Read the thread below from beginning to end:
http://www.diyaudio.com/forums/solid-state/25438-phase-gain-plot-will-oscillate-9.html
I could be wrong, but there is only one loop-gain in the amplifier. The only stability that you should worry is output that feedback in diff pair (retro feedback) if there is a phase inversion circuit is unstable as you already should know.
I already had problems with minor Loop (was not linear) the amp was a diff Vas, which gave problem for high frequency. but the problem I saw in Global Loop Gain.
I already had problems with minor Loop (was not linear) the amp was a diff Vas, which gave problem for high frequency. but the problem I saw in Global Loop Gain.
Thank you jcx for the links. The second one looks good... Shall print them out at work tomorrow.
Damn, I used to be able to do all this s domain stuff. That was back in 98. Now I have an interest that requires relearning (and actually understanding the underlying principles) all this stuff.
Its mad you end up studying harder for a hobby than for a qualification.
Only 98? Try 78!
Gosh I'm old. I think I need a nap. 🙂
I'm not sure how useful it will be, but the simple Tian-type loop-breaker implementation, for LT-Spice, was posted at
http://www.diyaudio.com/forums/chip-amps/107246-dc-servo-question-10.html#post1298611
There is a zip file that you can download, after the schematic. You can just copy and paste the loop-probe portion of the schematic into your own LT-Spice schematics. It has a relatively-easy way to change between open loop and closed loop simulation. The important part (the plotted expression) is in the .PLT file that sets the plot settings.
There is also a quick description of the method's setup at
http://www.diyaudio.com/forums/solid-state/115945-leach150-phase-margin-problem.html#post1409242
I thought that I had posted some much better examples of the loop probe with LT-Spice. But I can't find them, now.
Hi everyone, thank you very much mcd99uk for opening this thread,
Sorry if I'm interrupting, I want to know how to probe the main nodes of the circuit, it was told me that I can use Tian probes to do this, but I don't know how to do this, I don't know how to isolate the main nodes of the circuit. Can you help me?
Best regards,
Daniel
Sorry if I'm interrupting, I want to know how to probe the main nodes of the circuit, it was told me that I can use Tian probes to do this, but I don't know how to do this, I don't know how to isolate the main nodes of the circuit. Can you help me?
Best regards,
Daniel
You're not interrupting anything...
I'm still trying to work this out 🙁
It would appear to be fundamental knowledge in the amplifier design business.
I'm still trying to work this out 🙁
It would appear to be fundamental knowledge in the amplifier design business.
There is a discussion of this in the TPC vs TMC thread that may help.
The problem finally became a lot clearer to me recently, so some of my earlier posts are not as perceptive as they could be.
There can be several.
Often there is effectively only one.
Read the LOOPGAIN2 example in the LTspice EDUCATIONAL directory.
The complicated expression that you plot does the calculation so you do not have to "isolate the main nodes"
Correct placement of the Tian probe is sufficient.
I wrote a few posts about this, should be easy to find with a search.
The next issue of Linear Audio will have an article that touches on this.
The issue after that will cover it in detail.
Best wishes
David
Hi everyone, thanks for your help,
I'm sorry I'm now doing a college work, I will repply later with news about the simulations.
Best regards,
Daniel
I'm sorry I'm now doing a college work, I will repply later with news about the simulations.
Best regards,
Daniel
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It's the returns ratio bit that's getting me at the moment and how the other loops interact with the loop you are interested in. Understanding will come eventually. Sometimes I have to stubbornly work at something until the concept clicks.
Funny you should mention Linear Audio. Bought Vol 6 today. It looked to have some interesting info. Just what I want to try and learn about at present. By the sounds of it vol 7 and 8 will have to be procured as well in the future.
Paul
Samuel Mason wrote the classic work in this area of feedback theory.
It was his thesis at MIT and is in the public domain and available on the web.
I need to read it a few more times, the maths is almost deceptively minimal but there is a lot of depth.
I hope you like the introductory instalment.
Best wishes
David
Hi everyone,
thank you very much for your help,
I've made a few experiments with the Tian probe, the circuits are attached, I don't now how to interprete the results, can you help me?
I don't even know if I'm doing it right 😕
PS: Why Michael Kiwanuka was banned from diyaudio? 🙁
Best regards,
Daniel
thank you very much for your help,
I've made a few experiments with the Tian probe, the circuits are attached, I don't now how to interprete the results, can you help me?
I don't even know if I'm doing it right 😕
PS: Why Michael Kiwanuka was banned from diyaudio? 🙁
Best regards,
Daniel
David,
Thank you for the reference. Shall hunt this out and add it to my study material. It's like doing a second job. All this research and in parallel finishing off my first proper project. Almost a recluse these days. 😀
The overview looks very good, looking forward to it turning up. 🙂
Is that an article in it by you?
Paul
Hi everyone,
Could you please take a look at my Tian probe LTSpice test circuits in my previous post, I don't know if I'm placing the probes at the right places, and if I need to "disconnect" the global NFB when I'm probing internal nodes, I also don't know how to interprete the results, but they look ok to me, the peaking at frequencies of less than 10Hz is weird.
I'm interested in Samuel Mason's thesis about feedback, can you give me the link?
Best regards,
Daniel
Could you please take a look at my Tian probe LTSpice test circuits in my previous post, I don't know if I'm placing the probes at the right places, and if I need to "disconnect" the global NFB when I'm probing internal nodes, I also don't know how to interprete the results, but they look ok to me, the peaking at frequencies of less than 10Hz is weird.
I'm interested in Samuel Mason's thesis about feedback, can you give me the link?
Best regards,
Daniel
Hi everyone,
I've made a few changes to the circuits, can you take a look to see if I'm doing something wrong?
What I can conclude with this results?
Best regards,
Daniel
I've made a few changes to the circuits, can you take a look to see if I'm doing something wrong?
What I can conclude with this results?
Best regards,
Daniel
Yes. It is the introduction to a series on advanced amplifier techniques.
Next one looks at TPC, TMC, Output Inclusive and similar compensation schemes.
One after that is intended to cover how to check the amplifier, with simple V probes, Middlebrook 1975, Tian et al and Middlebrook "Final" probe techniques.
I plan to continue after that but better to write first and announce latter.
I can say objectively that the rest of the issue is definitely informative, I hope you like my piece too.
Best wishes
David
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