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Design >> Analog Design >> Two stage ring oscillator
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Message started by raja.cedt on Feb 28^th, 2015, 10:50am

Title: Two stage ring oscillator
Post by raja.cedt on Feb 28^th, 2015, 10:50am

Dear all,
Can any one explain how can a two stage oscillator works, basically I didn't understand how it is satisfying Barkhausen criterion. Attached is a sample ckt diagram with each stage being designed by diff pair with cross coupled pmos load?
I can only think of ideal integrator as delay cell so as to satisfy barkhausen criterion with two stages
if I replace cross coupled load with resistive load, will it oscillate?

Thanks,
Raj.

Title: Re: Two stage ring oscillator
Post by newic on Mar 4^th, 2015, 5:05am

hi

each stage provides 90 degree. The total shift will be > 180o with the help of negative R by the cross-coupled.

Title: Re: Two stage ring oscillator
Post by newic on Mar 4^th, 2015, 5:06am

it will not oscillate if you replace with R load. It can also oscillate with inductive load.

Title: Re: Two stage ring oscillator
Post by raja.cedt on Mar 4^th, 2015, 9:06am

Hi,
Thanks for the reply, could you please explain how -ve resistance help to get phase 90deg from each stage. From simple small signal model, this stage boils down to a diff pair with -ve resistance, means -ve phase shift but still can each stage contribute 90deg?
For me looks like diff between resistive load and cross coupled pair is sign only(from small signal)---is this correct?

Thanks,
Raj.

Title: Re: Two stage ring oscillator
Post by newic on Mar 4^th, 2015, 11:23pm

you can refer to:
http://www.seas.ucla.edu/brweb/papers/Journals/A&RMar01.pdf

Title: Re: Two stage ring oscillator
Post by raja.cedt on Mar 5^th, 2015, 5:39am

Hi newic,
Good paper, but he is also using active inductor and making sure each stage contribute 90deg. In-fact with an inductive load single stage also suffices (ex--lc vco). But in the current scenario can we approximate cross coupled load with inductor, from small signal model I have got -ve inductor (similar to Gyrator except a polarity diff).

Any comments, sorry for asking many times, some how I am confused here..

many Thanks,
Raj..