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Homework Help: Op Amp Circuit Weird Characteristics

  1. Sep 12, 2016 #1
    1. The problem statement, all variables and given/known data
    New Doc 28.jpg

    2. Relevant equations

    3. The attempt at a solution
    I have a similair problem to this one in my notes and I know how to solve this circuit, but I am confused on some parts of it. First, at V1 why can we say the voltage there is 0V? Is it because it is connected to the ground (-) of the op amp and we can say that the potential is 0V at that point? Also, why is the op amp drawn like that? Why is the (+) on the bottom go to ground? What does that mean exactly?

    And V2 is also equal to 0 but I am not sure why. Is it for the same reason I said above?
  2. jcsd
  3. Sep 12, 2016 #2


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    Staff: Mentor

    We just approximate it to zero volts, because in comparison with the level of other voltages in the circuit it must be very much smaller. We know it is not exactly zero volts, but we know it will be very small so for the purpose of easy analysis we can regard it as approximately zero. This is based on the principle of the virtual earth at the ideal OP-AMP's inverting input, and google will help you with this.
    Last edited: Sep 13, 2016
  4. Sep 12, 2016 #3


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    Science Advisor

    Can you state "the ideal op amp approximation"? This will probably answer your questions. If you don't know it, it is probably in your notes or your textbook.
  5. Sep 12, 2016 #4
    Hmm ok. I'll do some googling later then.
    It is not unfortunately. So are allthe things I mentioned in my posts just givens about ideal op amps? Things we can assume at the start of the problem?
  6. Sep 13, 2016 #5


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    Homework Helper
    Gold Member

    Not really givens but rather the consequences of the properties of an ideal op amp. For example if the gain is very very large then for the output to be in a linear region the difference between + and - input must be very small, virtually zero. For example if the circuit is powered by 5v rails and the gain is 1,000,000 then the input must not differ by more than 5/1,000,000v. Otherwise the output will be limited by the supply rail and hence not operating in a linear mode.
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