Hello, I have a question to all who are involved in electronic circuits.

However, the question is not techical but more or less linguistic and concerns the general speech comprehension.

Problem description: There are some configurations (I avoid the term „electronic circuits“) that exhibit oscillatory properties during transient simulations (in fact: they show self-sustained sinusoidal oscillations). However, only if IDEAL opamps are used. Therefore, only simulations can reveal threse properties.

As soon as real opamp models are used (at least one pole in the open-loop frequency response) there are no oscillations.
Instead, the amplifier output saturates immediately, because the mentioned amplifier pole is shifted to the right half of the s- plane (RHP).

The problem touches the validity of the well-known oscillation condition (Barkhausen) - and the question is simply:

Based on the general speech comprehension - should such a special configuration be called „circuit that is able to oscillate"? Even, if this is the case for ideal (artificial) models only?

Thank you
buddypoor (LvW)

loose-electron wrote on Nov 29^th, 2011, 3:50pm:


I am afraid that I have expressed myself not clear enough.
Therefore, I will reformulate my question:

1.) Assume that  there is an electronic circuit (hardware) that is not able to oscillate - in spite of unity loop gain  at one frequency f=fo only. This is neither a surprise nor a failure of Barkhausen’s criterion because this rule is only a necessary one.  
2.) Surprisingly, if this circuit is transferred to a simulation program and if an ideal opamp model (gain not frequency dependent) is used, the output shows a sinusoidal signal having a frequency fo. (By the way, I can explain this fact - but that is not my question).

Now the question:
How should I describe the property of that specific combination of passive and active parts with respect to the oscillation condition?

Is that circuit an oscillator in accordance with Barkhausen’s criterion: Yes or no?

Thank you

You may want to look at this:
http://web.mit.edu/klund/www/weblatex/node4.html

To me, this all goes back to control systems and Blacks Law.

loose-electron wrote on Nov 30^th, 2011, 11:14am:


Loose-electron, thank you for this link to a "funny" contribution from K.H. Lundberg (2002). I know this text since several years - and I also have given to Lundberg some comments and corrections.
But as it seems he sees no necessity to modify his conclusions like the first and the last sentence:
"The  Barkhausen Stability Criterion is simple, intuitive, and wrong."
"Down with Barkhausen"
Even the headline is wrong: Barkhausen never has formulated a "stability criterion".
Perhaps he likes such provocative formulations - even when the are wrong.
To me he has (a) never read the original text from Barkhausen and (b) never heard about the difference between "necessary" and "sufficient".
None of these terms is even mentioned in his article.
In the mean time (within the last 3...4 years) a discussion took place in the magazine "Analog Integrated Circuits and Signal Processing" dealing with the validity of Barkhausen's condition (based on some so called "counter examples" introduced by V. Singh). For my opinion, there is a wide agreement that Barkhausen has formulated a necessary condition only (I can confirm this as I have his book on my desk).
Besides this, of course Black's formula is the basis for all calculations of feedback systems. And - as I have mentioned - I have no problems to explain the differences between the behaviour of both alternatives (ideal vs. real opamp model). My only concern is: Does it make sense to investigate per simulation a "circuit" based on an ideal opamp that never can be build - and that behaves completely different if real amplifiers are used?
Thank you    

hello,
@ alexder: can  you please tell me where to study about moving poles (i donno any thing about moving poles)

@ buddypoor: hello buddypoor, you have written "Surprisingly, if this circuit is transferred to a simulation program and if an ideal opamp model (gain not frequency dependent) is used, the output shows a sinusoidal signal having a frequency fo", is really worked, i guess you have to add some passive ckt which makes gain drops at least and infact with gain dependent amplier also works because after all you are having some gain drop in the opamp so you will reduce the phase shift required by the passive network. Correct me if i am wrong.

do you have the book which decribes counter examples? please tell me the exact title so that i can read.

Thanks,
Raj.

Hello buddypoor,
what i mean to say is when you have ideal opamp without any frequency dependent gain, there should be some netwrok in the loop which reduces the loopgain to 1 at some frequency.

regarding articles, please send me if you can otherwise please tell me the name so that i can find some otherway to download.

Thanks,
Raj.

loose-electron wrote on Dec 1^st, 2011, 2:13pm:


Hi Loose-electron.
In principle, I agree with all of the above.
But I think my description of a specific case (my first posting opening this thread) can serve as an example that your explantions/justifications of idealized models do not always apply.
As I have reported, there are circuits that show a behaviour that is strongly dependent on the amplifier model used for simulation (ideal or real). It is obvious that this cannot lead to a "better understanding" of the circuit and it's function.
On the other hand, if one is able to understand the reason for the observed phenomena, this certainly will enlarge the knowledge of system theory and related areas.    
Perhaps it's useful to give you a very simple example:
Try to simulate an IDEAL opamp with positive resistive feedback (10k/1k).
Of course, such a circuit will not work as an amplifier. However, all simulations (OP, AC, DC, TRAN) will result in a stable inverting amplifier with a gain of 20 dB. Does this lead to a better understanding of amplifiers?
And it is a fact, that the simulator did not make any error at all. The result is correct!

buddypoor wrote on Dec 2^nd, 2011, 1:33am:


This example is useful in understanding the question in your initial posting.

I would not refer to this ideal opamp with positive feedback circuit as an amplifier because it is only functioning as an amplifier because it is excessively idealized.