Stability in feedback circuits

[Harvs]

Hey All,

Could anyone point me in the direction of a good text on opamp circuit stability? 

I find I'm using more and more analog circuits that have marginal stability issues, and I don't really have a good solid grasp of both the theory and practical issues.

An example would be the electronic load, that when build in a similar manner to Dave's circuit (but with whatever opamp and MOSFET I had at the time), had serious stability issues.  I resolved that through a case of trail and error, but it would be nice if there was some analytical methods of predicting where there'll be issues.  I'm guessing that once you understand the fundamentals behind it, then most of the time there will be some obvious solutions to the problem.

[Rerouter]

it is quite hard to be analytical with it, in that every op amp and active device has a unique model, each with 50 or so tolerances from manufacture,

though there are a few small ways to always stablise circuits, each has there downfalls,

a small capacitance (pF)  between the output and inverting input will slow down your op amps slew rate, which for some fast loops can increase stability

for capacitive loads, e.g. a large powerfet, a buffer resistance will help stability by making sure the op amp doesnt saturate at its outputs from current draw.

there the basics and 90% of the time outside of simulation those 2 tweaks are all that is needed,

 

[penfold]

Hi

I came across a few articles by Analogue Devices which I found quite handy one of which is:
http://www.analog.com/library/analogDialogue/archives/35-02/avoiding/index.html

Analog also have a good series of videos, which are on youtube, one of them is


Or if you prefer the old fashioned way, I quite like Microelectronics by Millman and Grable (second edition) I know its a bit dated but all the theory hasn't changed much, there's a few of these books on Amazon uk for less than 5GBP

[Mechatrommer]

Stability Analysis Of Voltage-Feedback Opamp http://www.ti.com/lit/an/sloa020a/sloa020a.pdf
Op amp stability and input capacitance http://www.ti.com/lit/an/slyt087/slyt087.pdf
Handbook of Frequency Stability Analysis http://tf.nist.gov/general/pdf/2220.pdf
http://www.newtons4th.com/wp-content/uploads/2010/03/TestingLoopStabilityv2.pdf
google more i believe you can find more.

[Harvs]

Thanks for that, a useful set of articles there.  Particuarly

I came across a few articles by Analogue Devices which I found quite handy one of which is:
http://www.analog.com/library/analogDialogue/archives/35-02/avoiding/index.html

and

Stability Analysis Of Voltage-Feedback Opamp http://www.ti.com/lit/an/sloa020a/sloa020a.pdf

I'd watched the analog devices videos before, unfortunatly I feel they skipped too much detail in order to get it in the short timeframe.

Rerouter, you are completely correct that it will not be possible to carry out traditional circuit analysis and I'll keep those tips in mine.

[andyturk]

Analog also have a good series of videos, which are on youtube, one of them is
...

Off-topic question: How did they make this video? It looks like the speaker is writing on a plane of glass in between himself and the camera. Normally, this would make the writing appear reversed, but not here.

[GK]

I'd say they just did a horizontal flip on the video. Either that or the presenter has had a lot of practice writing backwards!

[andyturk]

I'd say they just did a horizontal flip on the video.

Yup, that's gotta be it. Might make an interesting upgrade to "DaveCAD".

[PuterGeek]

I'd say they just did a horizontal flip on the video. Either that or the presenter has had a lot of practice writing backwards!

That is indeed how it was done. You can tell because his shirt is buttoned on the wrong side.

Either that or he is wearing a women's blouse. 

[SeanB]

I thought that and had to check on a shirt to be sure..........