unstable opamp ringing oscillating and such

[panfilero]

can anyone in here explain why a OpAmp could oscillate or become unstable if there are 2 poles before it reaches 0dB on a plot of gain vs frequency?

Here's what I've gathered thus far... there's a cap internal to the op-amp, that cap is there for some reason and it puts the dominant pole in the op-amp's transfer function. That pole also puts a 90deg phase shift on the output of the opamp...

I think the phase shift is with respect to the input signal, so now output is 90deg out of sync with the input

adding another pole to the transfer function would cause our gain to drop by another 20dB/decade (or 6dB/Octave) and also another 90deg shift on our phase... so now we're 180deg out of phase with respect to our input...

but why would this be bad?

Is there a way to explain this where it makes intuitive sense? or is this something that can only be seen coming out of the math in a non-intuitive kind of way?

much thanks!

 

[crutschow]

There is a 180 degree phase shift between the inverting (-) input of an op amp and its output. If you have feedback from the output to the inverting input with another 180 degree shift, then the total shift is 360 degrees, which means the output is in phase with the input, giving positive feedback. This can generate oscillations, ringing, and such.