[sdiy] Taming an oscillating unity-gain voltage follower

[Tim Stinchcombe]

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