Equivalent input capacitance

Discussion in 'Homework Help' started by psycoadam, Oct 18, 2011.

  1. psycoadam

    Thread Starter New Member

    Oct 18, 2011
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    [​IMG]I am having difficulty with understanding how to solve the circuit with the capacitor tied to the output of the op-amp. I assume that I break the circuit down into two parts and then solve for the Thev. equivalent circuit but with no input voltage I am at a loss. Do I add a source at the output and then open circuit the capacitor and solve the circuit that way?

    any help would be appreciated.

    the problem asks for the equivalent input capacitance Ceq.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    The first op amp is a follower/buffer. The second op amp is an inverter with gain. So what effect would this have on the apparent capacitance caused by C at Ceq? Remember C = Q/V.
     
  3. psycoadam

    Thread Starter New Member

    Oct 18, 2011
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    So if the gain of the follower is one, and the gain of the inverter is R2/R1 that leads me to
    Ceq = C(1+Av)? where Av = R2/R1?
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Close. This is not the same as Miller capacitance.

    EDIT: It is the same as Miller capacitance. A thousand pardons.:(
     
    Last edited: Oct 19, 2011
  5. psycoadam

    Thread Starter New Member

    Oct 18, 2011
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    I guess I am trying to make this harder than it really is. I understand the C=Q/V but what is confusing me is the charge. I thought of using a capacitive divider, but in the end I get a negative value for Ceq because in the problem C = 6nF, R1 = 3.8k, and R2 = 24.8k.

    Vo/Vin = C/(C+Ceq)

    and since Vo/Vin = R2/R1 then

    Ceq = C* (R1-R2)/R2

    Thank you all for your help
     
    Last edited: Oct 19, 2011
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I am very embarrassed.:(
    You were correct in post #3. It is the same as Miller capacitance. I'm sorry for confusing you.

    Carl, I needed you to keep me honest. Where were you?:eek:
     
  7. crutschow

    Expert

    Mar 14, 2008
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    Sorry, by the time I saw your post your had already corrected it.:rolleyes:
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    OK, you're forgiven. I hope I am also.:(
     
  9. psycoadam

    Thread Starter New Member

    Oct 18, 2011
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    Thanks guys! I really appreciate all the help! Don't worry, I was just making sure I was not going insane :)

    Miller Capacitance it is!
     
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