Phase-Shift Oscillator

Discussion in 'Homework Help' started by MrP90, May 28, 2013.

  1. MrP90

    Thread Starter New Member

    May 19, 2013
    12
    0
    Hello!

    I have to find the minimum required value of Rf for oscillations to start in this circuit. R=1k e C=80nF

    [​IMG]

    I know that for oscillations to start, Vo/Vf has to be over 29. But I can't find that expression.

    I wrote these equations:

    (V1/R)+(V1-V0)*(sC+1/Rf)+(V1-V2)sC=0

    V2/R+(V2-V3)sC+(V2-V1)sC=0

    Vf/Rf+(V3-V2)sC=0

    Since the ampop is ideal, I admitted that v-=0, so the third resistance is connected to the ground through the virtual short circuit.

    What am I doing wrong?

    Thank you for your help!
     
  2. The Electrician

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    Oct 9, 2007
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  3. t06afre

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  4. MrP90

    Thread Starter New Member

    May 19, 2013
    12
    0
    Thank you so much, I'll check those websites to see if I can get to the solution :)
     
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