Re: OTA - C oscillator
push_du said:
It is a paper by Dr. Senani from where I used the circuit. Kindly tell me what problem is persistent in my way of simulation. Thanks.
Hi, push_du
1.) At first, a more general comment:
I would be very surprised if the circuit as given by you would originate from Prof. Senani, because he is a very experienced analog "guru".
And I am right: The paper you have mentioned is not from Senani, but from M. T. ABUELMA'ATTI who claims to have simplified and improved Senani`s circuit.
This is a good example proving that even contributions in
Electronics Letters are not always correct.
2.) So, what is wrong with the circuit? It's loop gain is unstable by itself! This can be easily verified by inspection (positive feedback of one OTA stage) and by simulation in the time domain. From the system theory viewpoint: The loop gain has a pole in the right half of the s-plane.
3.) And what about the oscillation criterion? As you have mentioned, the Barkhausen criterion can be shown to be fulfilled (result of an AC analysis for the loop gain).
This example shows a very important and very interesting fact, which is NOT mentioned in most textbooks:
A circuit with feedback which fulfills the Barkhausen criterion will NOT necessarily oscillate! There is a common misunderstanding about this criterion, since it is believed that it is a necessary as well as a sufficient oscillation criterion. This is not true!
Barkhausen never has claimed that a circuit with a loop gain of unity at one frequency will oscillate at this frequency. The other way round: He has stated that a circuit to operate as an oscillator must have a loop gain of unity. That´s the big difference between "sufficient" and "necessary".
I am afraid, that M. T. ABUELMA'ATTI was not aware of this.
Comments are welcome! Especially, I am interested to learn if anybody has seen a book which contains a
sufficient oscillation criterion .
Regards to all
LvW