Electronic – Op-Amp output inconsistent with theory

negative feedbackoperational-amplifierpositive-feedback

enter image description here

I was just playing around with a few Op-Amp circuits, and suddenly got stuck on this simple one.
As the circuit shows, the Op-Amp output is connected to its non-inverting input terminal, so it should NOT act as a voltage buffer. However, as my image shows, its a different story (simulated with NI Multisim 14.0, however I got same results using EveryCircuit) We have virtual short. Output is 5V.
I expected -15V (Inverting input terminal would rise, differential voltage would go -ve, output would rush to -ve rail, and differential voltage still stays -ve).
I also cannot understand the output frequency of 50kHz. Why would it oscillate with such a small peak-to-peak voltage? I can't justify this observation. Is this a problem with the simulation software?

Best Answer

No - it is not a problem of simulation software. Surprisingly, the result is correct - but not realistic. What does this mean?

Theoretically, the result would be correct under two environmental conditions:

  • No power switch-on transients (power available since (t-infinity))

  • No external disturbances (noise, etc).

During simulation, the program automatically assumes these two conditions - unless you specify other conditions.

However, if you would switch-on both (or one) supply voltage at t=0, a transient analysis would reveal that the circuit is saturated due to positive feedback.

The situation can be compared with a mechanical model: A system of two balls - one lying upon the other one - could be stable as long as there is absolutely no external disturbance. But this is NEVER the case. The same applies to the real opamp usage: Power switch-on transients, supply voltage is not absolutely constant, noise,...)

I think, in 99.9% of all cases, the simulator does not fail but the user (misinterpretation of the results)

Comment: But there are two indications that the simulation result is "problematic" (not wrong, because the simulator was correct) and should be investigated in detail (under real conditions):

  • In your simulation, the output is positive - even with an input at the inv. node.

  • An AC analysis would show a rising phase function (which is also not realistic).