Re: Chaos in FF metastability

Daniel Lang <invalid@xxxxxxxxxxxxxxxxxxx> wrote:

If you have studied Op-amps, which have negative feedback instead
of positive feedback, you are probably familiar with the bode plot.

Yup. Studied, used, taught about :-)

The key to stabilizing the op-amp is to insure that the feedback
gain (open loop gain/closed loop gain) drops below unity before
the additional phase shift reaches 180 degrees.

Indeed. That's a nice simple explanation that's often missing
from the textbooks.

A similar analysis applies to flip-flops. The main difference
is that here the feedback is positive (-360 degrees) and so
latching to one side or the other is a stable state. If
you get an additional -360 degrees of phase shift before
the open loop gain reaches zero, the device can oscillate
at that freqency if stimulated properly.

OK, thanks for that insight - the (rather obvious) point I'd
missed was that you need an extra *360deg* of phase
shift to get the thing to oscillate, not 180deg.

2 gain stages with single pole RC roll-off will have a maximum
additional phase shift of -180 degrees. Cascading 4 or more
gain stages will usually ensure that the phase shift exceeds
-360 degrees before unity gain is reached due to the presence
of higher order poles & wire delays.

OK. Thanks for the nice and appropriate description.
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