some doubts in oscillators

[iVenky]

I have some doubts in oscillators.

In colpitt's oscillator we use this tank circuit



This is clearly a low pass filter. But we should have band pass filter in the feedback path for an oscillator to select its frequency.

How does this provide a 180 degree phase shift (another 180 degree phase shift is provided by the CE configuration) so that we could satisfy the Barkhauson criteria?

Thanks in advance.

 

[LvW]

This is clearly a low pass filter. But we should have band pass filter in the feedback path for an oscillator to select its frequency

How does this provide a 180 degree phase shift (another 180 degree phase shift is provided by the CE configuration) so that we could satisfy the Barkhauson criteria?
Thanks in advance.

At first, a bandpass is not needed in the feedback path (remember the classical phase shift topologies).
Secondly, the lowpass function of the shown feedback circuitry (with a finite source resistance) is of 3rd order and, thus, crosses the -180 deg line.
Therefore, the Barkhausen condition can be fulfilled at one single frequency.

 

[iVenky]

At first, a bandpass is not needed in the feedback path (remember the classical phase shift topologies).
Secondly, the lowpass function of the shown feedback circuitry (with a finite source resistance) is of 3rd order and, thus, crosses the -180 deg line.
Therefore, the Barkhausen condition can be fulfilled at one single frequency.

Yes. I understood. Could you please help me in finding out the transfer function of this circuit?

Thanks in advance

 

[LvW]

Can be found here:
https://www.seas.upenn.edu/~ese319/Lecture_Notes/Lec_1_Introduction_09.pdf

---------- Post added at 14:46 ---------- Previous post was at 14:39 ----------

Correction:

Sorry, the above link is not correct.
Try this one:

https://www.seas.upenn.edu/~ese319/Lecture_Notes/Lec_19_Colpitts_Osc_10.pdf