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Simple ideal op-amp

Discussion in 'Electronics Homework Help' started by kimbokasteniv, Mar 21, 2012.

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  1. kimbokasteniv

    kimbokasteniv

    6
    0
    Jan 26, 2012
    So I have the circuit below:
    [​IMG]

    And I have the equation which I believe describes the circuit, as well as the same equation solved for Vo:
    [​IMG]

    But, if you try the numbers in my solved equation you will find that they do not agree with the values multisim has calculated. I guess that means my original equation is wrong or I messed up my algebra somewhere. However, I have been looking at this for over an hour and have been unable to see anything peculiar.

    Any suggestions on where I messed up?

    Oh, and Vcc on the op amp is 15V.
     
  2. GreenGiant

    GreenGiant

    842
    6
    Feb 9, 2012
    First off ideal implies that there is no Vcc for the op-amp, it will give you an exact amplified output based solely on gain, no rail to rail or anything like that.

    Second, you may want to take a look at the voltage supplied to Probe 2

    Third, if this is a Differential amp you appear to be missing a resistor on the positive side of the amplifier.

    Where is the 404 coming from in the second formula?
     
  3. kimbokasteniv

    kimbokasteniv

    6
    0
    Jan 26, 2012
    Hey GreenGiant, thanks for the response.

    My book lists a Vcc votlage for the op-amp, and it also does suggest that it is in an ideal op-amp circuit. So I guess they are stating the Vcc just so I know what would be past saturation.
    The voltage at probe 2 is equal to the voltage at Vn, which according to ideal op amp rules is equal to Vp, and Vp is equal to 100mV.
    In my top equation I substituted Vn with 100mV. So, I believe that is all correct, was there something else you were getting at?

    As for the 404:
    [​IMG]
     
    Last edited: Mar 21, 2012
  4. GreenGiant

    GreenGiant

    842
    6
    Feb 9, 2012
    Ok, I was mentioning the Vcc because in Multisim an Ideal Op-Amp has no Vcc, it is limitless. it is referring to perfect gain.

    I get a slightly different result than 404, you may want to check your math

    Even so, I am not sure where you are getting these formulas, can you take a picture of exactly what your book shows for this problem?
     
  5. kimbokasteniv

    kimbokasteniv

    6
    0
    Jan 26, 2012
    Shoot, the coefficient of 0.1 should be 101. Those numbers work. That solves my problem, thanks GreenGiant.

    For the virtual/ideal op-amp in multisim, there is the swing voltage parameter, which I assume reflects a Vcc voltage.

    Otherwise, I got those formulas by doing KCL at probe 2.

    I can scan in the question if you are still curious though--I'm just away from home right now. Let me know.
     
  6. GreenGiant

    GreenGiant

    842
    6
    Feb 9, 2012
    I get where you are coming from now, I wasn't sure where you were at in the stages of learning about electronics, being such a simple circuit I thought you were very early on, but working with KCL and all that about circuit analysis it all makes sense now
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,473
    2,819
    Jan 21, 2010
    1/2k + 1/200k is (2k + 200k)(2k*200k), not (2 + 200)/(2k*200k)

    You've dropped a 1000's multiplier. And the value you get is 1/1980, not 404

    I can get 404 if I type it into my web browser though. :)
     
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