Ideal Op Amp

Discussion in 'Homework Help' started by cheypr, Dec 8, 2009.

  1. cheypr

    Thread Starter Active Member

    Oct 13, 2009
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    Greetings!!!

    I was doing my homework and I'm not sure about this problem. Is it OK or not? If not, what I'm doing wrong?:confused:
     
  2. steveb

    Senior Member

    Jul 3, 2008
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    I didn't check the entire thing, but I stopped when I got to the point that you said Vo=3V+. This is not correct. The negative feedback path from output to the negative terminal shows a feedback of Vo=2V-, and you are assuming that V+=V-.
     
  3. cheypr

    Thread Starter Active Member

    Oct 13, 2009
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    in my notes i have that in an ideal op amp V+=V-. That's why I'm assuming that 3V-=3V+.

    From my KCL@V- i get that V0=2V- + V-. So, Vo=3V-. What I'm doing wrong?

    Am I taking the right approach?
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
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    Then it comes to feedback system it is correct to assume that V+=V- But I did not quite understand why you set Vo=3V
    Do you have the correct answer?
     
  5. cheypr

    Thread Starter Active Member

    Oct 13, 2009
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    No, unfortunately I dont have it. I'm getting Vo=3V from the KCL@V-.
     
  6. t06afre

    AAC Fanatic!

    May 11, 2009
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    As I see V-=Vo*50/(50+100)
     
  7. t06afre

    AAC Fanatic!

    May 11, 2009
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    Also use superpostion to find the V+ that is simpler
     
  8. steveb

    Senior Member

    Jul 3, 2008
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    I'm sorry about my mistake above. I was running out the door to go to work, and didn't think carefully.

    Basically, you did the problem right.

    You have Vo=3*V+ and Vs=-(4/5)*V+

    Your mistake is your last step. If you combine these you should get.

    Vo/Vs=-15/4

    Time to slap your head, and say "doh"! :)
     
  9. cheypr

    Thread Starter Active Member

    Oct 13, 2009
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    DOHHHHHHH!!!!!!!! thanks to you all
     
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