Jess Chen
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I can't resist chiming in on this one. Stability has always been one of my favorite topics.
I agree with the reviewer. It is always a good idea to check stability with transient simulations. Kick the circuit in as many ways as you can think of and look for unstable behavior that was not predicted by your small signal analysis.
As for a more rigorous approach, large signal stability is essentially the stability of nonlinear systems. If the system were linear, small signal stability would suffice. As I recall from the last class I took on nonlinear systems, there is no one method that suits all systems. However, probably the most rigorous and most general approach is to use Lyapunov functions. You can think of a Lyapunov function as an expression of the total instantaneous energy stored in the system. If, with no independent dynamic sources, the energy always decays with time, the system is stable. Sounds reasonable but I've only seen it used in high level proofs. I've rarely seen it applied to practical circuits.
I usually think of two kinds of large signal instability: the existance of a stable but undesired operating point, and a limit cycle. If an undesired operating point exists, if you kick the system hard enough, it might be sucked into the undesired operating point. An undesired stable operating point can be found by performing small signal stability analysis over a wide range of DC operating points. A latching condition could be an undesired operating point. The limit cycle potential, (a large signal oscillation), can usually be assessed using describing functions. The describing function is a large signal transfer function. It is like a small signal transfer function except that it depends on the amplitude of the input sinusoidal drive. Most text books on nonlinear systems will have a section on describing functions and you could probably find descriptions on the internet. However, if you like, I can try to explain them.
Hope this helps.
-Jess
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