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Operation amplifier - inverting amplifier

  1. Aug 14, 2011 #1
    operation amplifier -- inverting amplifier

    1. The problem statement, all variables and given/known data


    Please refer my attachment which is inverting operational amplifier.
    Iin = (Vin - VA)/Ri
    If = ( VA - Vout) / Rf
    My question is why not
    Iin = (VA - Vin)/Ri
    If = ( Vout - VA) / Rf
    Thanks in advance revered members

    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     

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  2. jcsd
  3. Aug 14, 2011 #2
    Re: operation amplifier -- inverting amplifier

    Not sure if this is what you are getting at, but: Conventional current presumes positive charge carriers (because it was defined before we knew about electrons) and it became the defacto standard in literature. So unless otherwise noted, pretend flow goes from positive to negative.
     
  4. Aug 14, 2011 #3
    Re: operation amplifier -- inverting amplifier

    i dont understand sir
     
  5. Aug 14, 2011 #4
    Re: operation amplifier -- inverting amplifier

    (Although you've shown no arrow in the diagram indicating the direction of Iin, I've assumed it goes from left to right.)

    Since conventional current is defined as going from positive voltage to negative, Iin has a positive value when Vin is positive. So the first equations are correct (for conventional current). Google "conventional current" and "electron current". The second equations would be correct for ~electron current~ rather than conventional current, but as I said, conventional current is to be assumed unless otherwise stated.
     
  6. Aug 14, 2011 #5

    gneill

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    Staff: Mentor

    Re: operation amplifier -- inverting amplifier

    First you should define your current directions. Usually Iin would be taken as flowing INTO the input lead. The choice you make will determine what is the correct expression for the current that flows through Ri.
     
  7. Aug 15, 2011 #6
    Re: operation amplifier -- inverting amplifier

    Thanks for the replies gneill and fleem. yes the direction of the current is from left to right and how can it be construed as conventional current ( flow of positive charge carriers). I mean how u say that the current which flows from left to right is conventional current?
     
  8. Aug 15, 2011 #7
    Re: operation amplifier -- inverting amplifier

    Its like defining a coordinate system. If you define the X axis as pointing to the right, then a negative X velocity means travel is to the left.

    By "define a direction for the current", we mean define a direction for current when its value is positive. once you have chosen a direction that will be defined as "positve current flow", then you can tell the actual direction of the charge carriers according to the sign. For example, if you define left-to-right as "positive current flow", then you know that if its value is negative, then the charge carriers are moving right-to-left.
     
  9. Aug 22, 2011 #8
    Re: operation amplifier -- inverting amplifier

    Thanks Mr. fleem
     
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