[sdiy] Taming an oscillating unity-gain voltage follower

[Ian Fritz]

Compensation is well covered here:
http://focus.ti.com/lit/an/slod006b/slod006b.pdf
especially Ch 7.

  Ian




At 03:40 PM 3/16/2009, Tim Stinchcombe wrote:
>Hi Tom,
>
> > Can you prevent this oscillation by sticking a cap in to roll
> > off the
> > gain at high frequencies, in a similar way to the common practice on
> > inverting unity gain op-amps?
>
>Well I would say the answer to that is yes and no: 'yes' in that all the
>various methods suggested achieve the aim of suppressing oscillations; but
>'no' in that (as it appears to me) there is no simple fix like sticking the
>cap in for the inverting set-up, which is reasonably intuitive as the
>decreasing cap reactance reduces the gain as the frequency increases. The
>resistor between the op amp and the cap is probably the easiest method, but
>this and all the others to be seen mess around with the gain and/or
>introduce poles/zeroes which have the desired effect, but which (for my
>money at least) are a helluva lot less intuitive.
>
>I discovered that Pease book is on Google books, and page 102 with the
>method I used, fig 8.11b is in it:
>
><http://books.google.com/books?id=3kY4-HYLqh0C&pg=PA102&lpg=PA102&dq=pease+%
>22troubleshooting+analog+circuits%22+%22noise+gain%22+demichele&source=bl&ot
>s=opMSyq35oq&sig=pUrUt0FmalJEiGYMgYfPsyq0eAo&hl=en&ei=kcK-SbfCBOKHjAfFxqilCA
>&sa=X&oi=book_result&resnum=1&ct=result>
>
>I also found a number of different app notes which had one or more of the
>methods analysed, including adding the resistor either inside or outside the
>loop. It is frustrating that (so far) I haven't found a nice single
>reference that seems to cover all the methods from a coherent standpoint.
>This one though seems to be better than most (but I only started reading it
>over lunch, so I may yet be wrong...):
>
>http://www.st.com/stonline/products/literature/anp/14130.pdf
>
>None of those I have found cover that Pease configuration, so I guess I'm
>going to have to figure it out for myself. I'm hoping that at some point I
>find that I 'get it', as at the moment I just seem to be swimming around in
>a mass of formulae, which whilst I can understand each individual case, I am
>unable to see 'the bigger picture' and relate everything to everything else.
>Hopefully with a bit more application and effort I shall get there!
>
>Tim
>__________________________________________________________
>Tim Stinchcombe
>
>Cheltenham, Glos, UK
>email: tim102 at tstinchcombe.freeserve.co.uk
>www.timstinchcombe.co.uk
>
>
>
>
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