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Electronics Determining Op Amp Gain.

  1. Jan 18, 2017 #1
    1. The problem statement, all variables and given/known data
    Determine the Op amp gain of the following circuit.
    e4fd4417ef5239fdd5df6ff139ca5cb1.png

    V1 = 1v Vo = 4 V

    2. Relevant equations
    A = Vo/ VI

    3. The attempt at a solution

    Using voltage division I determined the voltage for the node at the +ve terminal

    VI = V1(1k/ ([1000 + 1]k)) = 1/1001 V
    Vo = 4v Given

    Therefore A = Vo/VI = 4/(1/1001) = 4(1001)

    However the answer is A = 4/(4/1001) = 1001

    So somehow I'm off by a factor of 4. I am really unsure why VI is 4/1001.

    Thank you so much for your time.
     
  2. jcsd
  3. Jan 18, 2017 #2

    Charles Link

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    The circuit does not make sense to me. Are you sure you don't have a feedback resistor somewhere? Also, are you sure the "-" terminal is connected to ground? Perhaps someone else can figure it out, but to me, it looks like it is missing something.
     
  4. Jan 18, 2017 #3
    I strongly felt the same way as well Charles. But this is a problem directly from my textbook and it has not been modified for the last 3 editions so I'm assuming it is correct. Thank you so much for the response though.

    EDIT: It is definitely very strange compared to the Operational amplifier problems I am used too. I am seeking enlightenment
     
  5. Jan 18, 2017 #4

    gneill

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    The circuit is okay, it's just that the op-amp depicted is not an ideal op-amp. Perhaps it would be better to think of it in terms of some general amplification stage with a finite gain. The idea is to find that gain given the "measured" input and output voltages.
     
  6. Jan 18, 2017 #5

    Charles Link

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    Taking the circuit as is, for a small positive input voltage, the output voltage would be pinned at +12 Volts or whatever bias you use for it. For a small negaive input voltage, the output would pin itself at -12 volts.
     
  7. Jan 18, 2017 #6
    Charles thanks for taking to the time to help out as well. I think gneill has the right idea of how to approach this problem so if we are to approach the problem assuming it is not an ideal OP amp what would I have to change in my solution?
     
  8. Jan 18, 2017 #7

    gneill

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    From what is given it appears to me that your result of A = 4004 is correct. Is the question complete as shown or were there other "characteristics" of this amplifier, such as input impedance?
     
  9. Jan 18, 2017 #8
    Yes you are absolutely right my A will be 4004 but the correct answer is indicated to be 1001 in all previous editions. the circuit provided is all information. The only additional information says Vo = 4 V and v1 = 1.0 V
     
  10. Jan 18, 2017 #9

    gneill

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    There is nothing provided in the given circuit to account for a factor of 4 other than the gain itself, so I'm thinking that there's an error in the book.
     
  11. Jan 18, 2017 #10

    Charles Link

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    I had difficulty reading the line in your OP where you have ## V1=+1.0 ## Volts and ## V_{out}=+4.0 ## Volts. I am glad that @gneill was able to help get the correct solution.
     
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