Need help in understanding input offset curent of Op-Amp

  1. Nov 30, 2013 #1
    [​IMG]
    This picture is from http://www.ecircuitcenter.com/Circuits/op_ibias/op_ibias.htm

    I am not very sure if I am getting the right idea of how the IB+ and IB- works, but when i solve it open up one of the IB to solve the problem.

    for IB+, (IB- is opened)
    Vout = V-( 1 + R2/R1), ..... V- = IB+R3
    Vout = IB+R3( 1 + R2/R1)

    for IB- (IB+ is opened)
    IB- = (V- - 0) / R1 + (V- - Vout) / R2
    Vout = V-(1+ R2/R1) + IB-....can i consider that when IB+ is opened, V+ = 0 = V- ?
    if I assume that I am right, i get what the page gives:

    Vout = -R2(IB-)

    one more thing, the IB+ is positive input biase current while the IB- is the negative input bias current? And the Iinput offset = difference of IB- and IB+ ->|IB+ - IB-| ?

    These two input bias current and input offset current are the parameters that always appear in the datasheet right?
     
  2. jcsd
  3. Nov 30, 2013 #2
    Try watch this video


    Also how can you add current into resistor ?
    Vout = IB+R3( 1 + R2/R1)
     
    Last edited by a moderator: Sep 25, 2014
  4. Nov 30, 2013 #3
    it is (IB+)R3(1 + R2/R1), sorry for the confusing writing.

    I consider that the IB+ current will pass throught the R3 and therefore the V+ is equivalent to (IB+)R3 . And i just apply the normal concept for finding the voltage amplification of operational amplifier circuit.
     
  5. Nov 30, 2013 #4
    For sure we can use a superposition to find Vout.

    First we assume IB+ > 0A and IB- = 0A
    So have

    Vout'1 = IB+*R3 * (1 + R2/R1) and this voltage is positive

    Next we open IB+ = 0A and we left with IB-.

    Vout'2 = - R2*IB- and for this IB current direction the output voltage is negative.

    And finally

    Vout = Vout'1 + Vout'2 = IB+•R3•(R2/R1 + 1) - IB-•R2
     
    Last edited: Nov 30, 2013
  6. Nov 30, 2013 #5
    No, in datasheet you will find the value for the average input bias current.
    IB = (IB+ + IB-)/2
    and offset current
    Ios = IB+ - IB-

    so

    IB+ = IB + Ios/2

    IB- = IB - Ios/2
     
  7. Nov 30, 2013 #6
    get it, thanks you very much !!
     
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