Harmonic Oscillator with ideal Opamp models

[FvM]

In the present circuit, an initial error signal isn't reduced but increases beyond bounds by feedback effect. That's not what we understand as conditional stability, I think.

 

[varunkant2k]

(What is the meaning of ... positive feedback is having infinite gain ??)
To example 1:
(a) Finite inverting gain (20 dB) for opamp ideal (no frequency dependence, without internal delay);
(b) instabilty (saturation at VDD or VSS) for real opamp model.

To example 2:
(a) Oscillations with fixed amplitude for opamp ideal; diode action: gain increase for rising amplitudes.
(b) instability (saturation at VDD or VSS) for real opamp model.
LvW

Hi LvW, Sorry for late reply, I was misplaced outside in the path of life journey .
Anyways, In response to previous discussion, I found this post very interesting and most of the things I am agree with all the loaded response. But always I see discussion lacks some basic device theory.

I am again here with some basic things with questions?
(What is the meaning of ... positive feedback is having infinite gain ??)
A: Positive feedback always gives infinite gain @ 0 frequency/infinite time. ( pole at the origin).
But in this process it may transform in to latch up (non linear) or oscillator ( critically stable: depends on +ve/-ve f/b)).
Check regenerative latch used for high resolution comparators. ( approaches infinite gain)

Now, Is stability analysis from criteria above for nonlinear devices possible? (I know modeling and blw blw.. )
My point: Example 1, 2 are unstable ( when positive feedback),
Reasoning:Ok from your side, Ex:1 is stable, then can you use it in your application. Off course you may not, You need closed loop with negative f/b.

So, I am not telling the all the results and behavior explained here are not coming.
They are there, My point is, these simulation behavior for ideal/real opamp not violating any of criteria.

 

[LvW]

Hi varunkant, welcome back.

I am sorry, but I don't know how to answer. What really is the question?

Quote: Ex:1 is stable, then can you use it in your application. Off course you may not, You need closed loop with negative f/b.

For example, what is the meaning of this sentence?

In this discussion it is important to mention if you are referring to ideal or real opamp models (because the fundamental behaviour of the examples depends on that).
To answer at least one of your questions: Classical stability analysis are for linear systems only. However, for loop gains with lowpass response there are approximate methods, which can take non-linearities into account (key words: describing function, harmonic balance, 1st harmonic analysis).