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Ideal opamp analysis

  1. Mar 9, 2015 #1
    Given this ideal op-amp, determine Vout if Vin = 2V and Vs = 15 V.
    So I know that the ideal op-amp has infinite gain. As the difference between V+ and V- is substantial (2V), wouldn't that mean the output would theoretically by infinitely large but instead max out at Vs? Am I understanding correctly, or am I way off?
  2. jcsd
  3. Mar 9, 2015 #2


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    Yes - you are right. Each application of opamps as linear amplifiers requires dc feedback. This allows a suitable dc operating point in the mddle region between the supply voltages as well as an amplification factor that is determined by the feedback resistors only.
    Otherwise, even very small input voltages cannot be amplified (in case of REAL opamps) because an always present offset voltage will drive the output into saturation.
  4. Mar 9, 2015 #3
    Okay, that makes sense. Thanks for the response!
  5. Mar 9, 2015 #4


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    Science Advisor

    The way it is drawn the output would be pinned at the -Vs rail, not Vs. The inverting input (-) is higher than the non-inverting input (+) here.
  6. Mar 9, 2015 #5
    So it should be -15 V?
  7. Mar 9, 2015 #6


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    Staff: Mentor

    Depends on the opamp. Some can drive their output to the rails, and others saturate somewhere close to the rails (within a volt or two). If you are to assume it is an ideal opamp, what do you think?

    BTW, is this from a schoolwork assignment? What is the context of the question?
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