Non Inverting summing amplifier

Discussion in 'Homework Help' started by naspek, Mar 18, 2010.

  1. naspek

    Thread Starter Member

    Feb 17, 2010
    Hey there...
    i've got question on how to derive the expression for the output voltage
    of non inverting summing amplifier...?
    i did search on the other website.. but.. they didnt show the step by step

    i know that
    Vo = 1/n (V1 + V2 ... +Vn)(Rf/R1 + 1)
    but.. how to get it step by step? apply KCL? VDR? :confused:
    Last edited: Mar 18, 2010
  2. rogs

    Active Member

    Aug 28, 2009
    If you use a dual opamp, then the calculation is simple. Simply connect the inputs to be summed to a unity gain inverting summing amplifier, and invert that output, with a unity gain inverter, using the second amp.
    Then the output voltage is simply the sum of the input voltages - V1 + V2 + V3... etc.
    Using an opamp in this way means that the inverting option for the first stage is a true 'virtual earth' summing junction, and you simply add the voltage inputs together. Can't see any point in making it any more complex than that?
  3. naspek

    Thread Starter Member

    Feb 17, 2010
    Apply superposition: Suppose only V3 is present so Vin = [R3/(R3+R2)]*V3
    Suppose only V4 is present so Vin = [R2/(R3+R2)]*V4
    Add these Vin = [R3/(R3+R2)]*V3 + [R2/(R3+R2)]*V4
    Vout = vin ( 1+Rf/R1)

    is this derivation correct?