December 21, 2014

Puzzle: Ring Oscillators

Ring Oscillators are very commonly used circuits in SoCs, where they find their use in Voltage Controlled Oscillators used inside the PLLs, and also used as Silicon odometer circuits- which are used to track various parameters of the device like variation of timing with ageing etc.

It is also a well-known fact that ring oscillators have odd number of inverters connected in form of a chain, as shown below:


And the frequency of oscillations is given by the expression:
f = 1/(2NT)
N= Number of inverters (odd)
T= Propagation delay of a single inverter

Can you comment on the below circuit? Maybe an expression for it's oscillation frequency? Or perhaps any analytical expression (without bothering about the intricacies of various transistor parameters) for the voltage? Assume the operating voltage to be 1V.


Please post your answers here. I'd be happy to share the solution and my thought process in a couple of days.


23 comments:

  1. Can you elaborate?

    ReplyDelete
  2. I surely will. However, I'd like to wait for a day or two to post it here. If you want, I can mail it to you.

    ReplyDelete
  3. Vout = Vin = Vdd/2..?? half the bias voltage..

    ReplyDelete
    Replies
    1. Yes, that is correct. No oscillator like function.

      Delete
    2. no, that's wrong.. read it again...

      Delete
  4. and not an oscillator like performance in achieved... I suppose

    ReplyDelete
  5. plz send to remesh.manu0@gmail.com

    ReplyDelete
  6. functionally both the circuits are equal ... bt the later 1 ... ie ., the oscialtor with single invertor has highest frequency compared to any other oscilators using invertors with the given propagation delay.


    I understood the above theory ... bt did not get vout=vin=vm ... pls elaborate in mail gautamireddy1990@gmail.com
    Plz comment or mail to gautamireddy1990@gmail.com

    ReplyDelete
  7. Hi Naman,

    Can you please elaborate how Vout equals to Vin when an inverter is established in between input and output

    ReplyDelete
  8. And how the propagation delay of an inverter effects the above circuit

    ReplyDelete
  9. jkam273@gmail.com
    please mail the explaination

    ReplyDelete
  10. for oscillation atleast three stages req. to satisfy Barkhausen criteria of oscilation. in which phase shift should above 180 degree. to get above one degree we need to connect three stages one inverter will give only 90. if we use two inverter 180 but even bcz of that no change in output and even formation of latchup.

    single inverter wont provide any oscillation

    ReplyDelete
  11. vout=vin=vm ???... pls elaborate this and bias generator in mail prashantguptaengg@gmail.com

    ReplyDelete
  12. Please mail me the explanation @ montu.bhavsar75@gmail.com

    ReplyDelete
  13. Hi,
    Please email me the solution on jaiswalbhaskar@gmail.com

    ReplyDelete
  14. hey,
    please emial me the answer to rathod.hari31@gmail.com

    ReplyDelete
  15. hi

    Please email me the solution peeyushtapadiya@gmail.com

    ReplyDelete
  16. If I go by control theory the second circuit is only having a single pole (not a pair of complex/imaginary poles) as required for marginally stable systems. Hence the system will always settle immediately to a steady state value (likely tobe closure to the 50% threshold between high and low. thus frequency is 0, i.e a DC.

    ReplyDelete
  17. Hi,
    Please mail me the solution.

    shethketan30692@gmail.com

    Thanks in advance..

    ReplyDelete
  18. i think it is about bias generator, but it requires atleast 3 inverters.

    please mail me

    contactsanjai483@gmail.com

    ReplyDelete
  19. Explanation can be - you have two different function ( 1st function VTC Char of inverter and 2nd function Vout= vin) applied on same inverter.so you have "one single solution".It means two different curve meet at one single point that is Vdd/2 if the inverter sizing correctly.

    ReplyDelete
  20. Explanation can be - you have two different function ( 1st function VTC Char of inverter and 2nd function Vout= vin) applied on same inverter.so you have "one single solution".It means two different curve meet at one single point that is Vdd/2 if the inverter sizing correctly.

    ReplyDelete