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Homework Help: Electronics OP-amp question

  1. Nov 17, 2008 #1
    1. The problem statement, all variables and given/known data
    You have an op-amp circuit, a very simple one... All it is is a signal produced (triangle wave) from a generator that is sent into both the inverting and non inverting inputs. what does the output wave look like??

    3. The attempt at a solution

    Im pretty sure this is very easy and i think i am looking into it too hard... but if you send an identical signal into both noninverting and inverting inputs of an op amp, wont there be no output signal because the amp will invert the signal and add it to the normal signal sent into the non inverting for a net of 0??? Or does that not matter, would i just come out as the signal times the amplification??? I guess im just not 100% sure about what inverting and non inverting do, thanks a lot guys.
  2. jcsd
  3. Nov 17, 2008 #2


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    Science Advisor
    Homework Helper

    Simple rule: An opamp changes the output to reduce the difference between the inputs.

    So you are correct it will have no output. Actually in the real world it will immediately lock into full rail +ve or -ve output becuase it has infinite gain and there will be some tiny difference in the input!
  4. Nov 17, 2008 #3


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    Staff: Mentor

    For an ideal opamp, yes, the output would be zero. But for real opamps, you would need to look at the CMRR (common-mode rejection ratio), and at the input offset voltages and currents, in order to provide a complete answer. Check out the datasheets for some common opamps to read about these parameters, and then post here what you think the full answer is...
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