Op amp problem please help!

Discussion in 'Homework Help' started by upopads, Mar 30, 2008.

  1. upopads

    Thread Starter Active Member

    Dec 18, 2007
    42
    0
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    This is not a "homework for pay" site. We'll help with advice and hints, and we'll confirm correct answers. But we will not do your work for you.

    First find Vo. Then what you have is an algebra problem.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Here's the circuit - clicking on the link seemed to take forever to come up!

    [​IMG]
     
  4. upopads

    Thread Starter Active Member

    Dec 18, 2007
    42
    0
    Is Vo just the inverting op amp equation (-Rf/R1)*Vs?
     
  5. upopads

    Thread Starter Active Member

    Dec 18, 2007
    42
    0
    No matter what I do I can't get Rf to cancel out correctly.
     
  6. upopads

    Thread Starter Active Member

    Dec 18, 2007
    42
    0
    I've been working on this for a while on paper and I've done it a numerous amount of incorrect ways. The answer simplifies to Vj = (R1-KRj)Vs.
     
  7. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    I contest that answer. If Rj approaches zero, then Vj will approach zero as well, and (R1-KRj)Vs will approach (R1)Vs.
     
  8. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    No. There is feedback on the + input as well as the - input.

    Remember: an op amp will produce an output as needed to bring the two inputs to the same voltage (if at all possible).
     
  9. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    I have succeeded in obtaining the answer that you indicated in your initial post.

    That answer is:

    V_{}j =V_{S}\left(\frac{1}{1-\frac{R_{1}}{K*R_{j}}}\right)

    The steps I used were

    1. Write the expression for the voltage at the opamp's negative terminal as a function of Vs, R1, Rf, and Vout.

    2. Write the expression for the voltage at the opamp's positive terminal as a function of Rf/K, Rj, and Vout.

    3 Set the expression from step 1 equal to the espression from step 2 and solve for Vout.

    4. Plug the expression for Vout from step 3 into the expression for the voltage at the opamp's positive terminal from step 2 and then solve.

    hgmjr
     
  10. upopads

    Thread Starter Active Member

    Dec 18, 2007
    42
    0
    That equation is written correctly, except for that Vout is supposed to be Vj. I'm still not quite getting it, i always end up with Rf at the end.
     
  11. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Greetings upopads,

    Thanks for alerting me to the error in my equation.

    If you can post the results of your attempt at step 1 and step 2 as described in my earlier reply, someone will help you with any difficulty you are having with getting those expressions correct.

    hgmjr
     
  12. upopads

    Thread Starter Active Member

    Dec 18, 2007
    42
    0
    Well I've been writing the first equation as Vn=-Vs(Rf/R1) and the second equation as a parallel combination of the two resistors Rj and Rf/K times Vo or simplified Vp=Vo([Rf*Rj]/[Rf+Rj*K]) and then I set Vp=Vn and solve for Vo but i can't get rid of the Rf term when i do that.
     
  13. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Unfortunately, the expressions you are using for Vn and Vp are not yet correct.

    I would recommend that you focus on the expression for Vp for the moment and temporarily replace the resistance Rf/K with say a resistor Rx. This just keeps things simpler until you have determined the correct expression.

    hgmjr
     
  14. Distort10n

    Active Member

    Dec 25, 2006
    429
    1

    The signal on the non-inverting pin is not feedback. The resistor RfK isolates the non-inverting input from the output of the op-amp.
     
  15. RmACK

    Active Member

    Nov 23, 2007
    54
    0
    Assuming the negative feedback is working correctly etc, we'll assume that V-=V+=Vj the inputs of the opamp are equal. -will check later.
    Then Vj=VoRj/(Rf/K+Rj) or Vo=Vj((Rf/(KRj))+1)
    (Vs-Vj)/R1=(Vj-Vo)/Rf
    We want Vj in terms of Vs so remove that Vo with equation given earlier:
    (Vs-Vj)/R1=(Vj-Vj((Rf/(KRj))+1)
    If this is correct, it should rearrange but that is not something I would like to try to do :eek:
     
  16. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    That's just plain wrong.
     
Loading...