At 05:41 AM 3/23/2011, Olin Lathrop wrote: >Dwayne Reid wrote: > > You can make most op-amps stable even with gain less than -1 by > > increasing the noise gain of the op-amp. > >Actually it's gains above -1 that are usually the problem, at least for >opamps generally tuned for stability. Specifically gains between -1 and += 1 >are the issue. Lower gain magnitudes decrease stability. > > > In other words, you add a > > gain-set resistor from the (-) input to ground. As long as the noise > > gain exceeds the minimum gain required for stability, the op-amp > > remains stable even is the input gain is less than that minimum. > >That is unpredictable since you're relying on characteristics of the opamp >as they differ from a ideal amp. The usual way to increase stability is t= o >add capacitance between the output and the (-) input. Some opamps, >particularly old one, have extra "compensation" pins for adding the >capacitor, but those are rare nowadays. Nope - gotta disagree with you here. Adding noise gain is completely=20 predictable. The technique is not well known but there is in fact=20 literature available that describes in great detail why it works. Regarding my comment about "gain less than -1" - I think that you=20 misunderstood what I was saying. Ignore the (-) in front of the=20 (1). In other words, I was saying that you can make a=20 unity-gain-stable op-amp stable even if the gain goes to -0.5 or even -0.10= .. That's why I used the words "less than". dwayne --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .