Bias current

Discussion in 'Homework Help' started by ECC83, Jan 6, 2013.

  1. ECC83

    Thread Starter New Member

    Jan 6, 2013
    13
    0
    Question:
    [​IMG]

    Here is the given solution:
    [​IMG]

    Now, the datasheet gives:

    Maximum offset voltage: 30mV
    Maximum input bias current: 60pA
    Maximum input offset current: 30pA

    I calculated the offset at Vout due to the offset voltage as 30mV the same as what the solutions give.

    I am a little confused about the offset voltage at Vout due to the input bias current however. I do not understand why he multiplied Ios_ by the parallel combination of the resistances.

    What I did was:

    [​IMG]

    Since the + terminal is grounded (superposition rule), we have a virtual ground at the node between the two resistors and since current cannot go into the opamp (golden rule).

    (Vout - 0)/450k + Ib- = 0

    Vout/450k = -Ib-
    Therefore Vout = -27uV due to the maximum bias current

    Whereas they seemed to use the total parallel resistance and get 2.7uV


    Please explain?
     
  2. WBahn

    Moderator

    Mar 31, 2012
    17,748
    4,796
    Think about this. You are trying to find the effect of current going into the op-amp inputs but are performing your analysis with the assumption that current can't go into the op-amp inputs.
     
  3. ECC83

    Thread Starter New Member

    Jan 6, 2013
    13
    0
    I do not quite get you?
     
  4. WBahn

    Moderator

    Mar 31, 2012
    17,748
    4,796
    What does it mean if there is an input bias current of, say, 1uA? It means that there is a current of 1uA going into each of the op-amp input terminals. If you are trying to determine the effect of there being an input bias current, how can it make sense to perform your analysis based on the assumption that there is no current going into the input terminals when that is precisely what a non-zero input bias current means?
     
Loading...