Opamp filter - oscillator

Discussion in 'Homework Help' started by xxxyyyba, Nov 19, 2015.

  1. xxxyyyba

    Thread Starter Active Member

    Aug 7, 2012
    283
    6
    Hello. Here is my task:

    filter oscillator.png

    How to solve this? I'm confused because of only one input in opamp.
     
  2. Brownout

    Well-Known Member

    Jan 10, 2012
    2,374
    999
    Not necessarily an opamp; it's just an amplifier with a gain of "A."
     
  3. xxxyyyba

    Thread Starter Active Member

    Aug 7, 2012
    283
    6
    We are studying opamps right now so it's probably opamp :)
    I started thread by mistake here, it should be in homework category...
     
  4. Brownout

    Well-Known Member

    Jan 10, 2012
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    It really doesn't matter what you think it is. It's a generic voltage gain block.
     
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  5. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    5,649
    1,595
    If it helps you visualize the circuit, think of it as a non-inverting opamp stage, with the R2-C2 node connected to the + input.

    ak
     
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  6. MrAl

    Distinguished Member

    Jun 17, 2014
    3,733
    791
    Hi,

    What you would do is first write out the transfer equation which of course includes the gain A, then solve for the A that makes one or more of the poles cross the jw axis into the right hand plane of the complex frequency plane. The input doesnt even matter that much but you probably assume that is a stable input, like a unit step.

    I have to assume that you know how to write the transfer equation or i could not see how this question could have been asked of you, unless you missed a lecture or something :)
     
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  7. xxxyyyba

    Thread Starter Active Member

    Aug 7, 2012
    283
    6
    To find loop gain, we cut feedback and apply test generator like this:

    dzeks1.png


    dzeks1.png

    Then we use conditions Re{Abeta} >=1, Im{Abeta}=0, to find A, right?
     
  8. xxxyyyba

    Thread Starter Active Member

    Aug 7, 2012
    283
    6
    I got A=6.7 ...
     
  9. Jony130

    AAC Fanatic!

    Feb 17, 2009
    4,174
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    Yes, your answer is good. But can you show as your work?
     
  10. xxxyyyba

    Thread Starter Active Member

    Aug 7, 2012
    283
    6
    Of course.

    0006.png

    0007.png

    0008.png
     
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