Re: [Help-gnucap] Unstable inverter isn't unstable

[al davis]

On Wednesday 05 March 2014, Orestes Mas wrote:
> One of my students made the obvious mistake of exchanging the
> 2 inputs of the  operational amplifier, thus simulating
> "de-facto" an Schmitt trigger circuit. But, to my surprise,
> the simulation results didn't show the Operational Amplifier
> saturation, but linear behaviour!

Welcome to simulation!

You have, without realizing, introduced an important concept in 
simulation!

What happened is that it found a "meta-stable" state.  It's a 
singleton in the response.

You think of that op-amp circuit as having two stable states 
(plus clip and minus clip).  You don't realize that there is a 
third metastable state midway between.  It is a point where 
there is a mathematical solution, but it is on a negative slope.  
Either side of that point, the solution moves away from that 
point.  It is the border between moving one way and moving the 
other way.

In a real circuit there is enough noise that before you notice 
it will go one way or the other.  In simulation, without noise, 
it could end up at exactly that point.

For another interesting experiment, try an AC analysis.  Look 
very closely at the results.