summing amplifier equation

Discussion in 'General Electronics Chat' started by Xufyan, Aug 9, 2011.

  1. Xufyan

    Thread Starter Member

    Aug 3, 2010
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    How to derive a general equation for a non-inverting summing amplifier , i have done this for an inverting but stuck for non-inverting as there are resistors on both the inputs ,
    what will be the equation of kcl ??

    why there is no derivation for non-inverting summing opamp in any book

    [​IMG]
     
    Last edited: Aug 9, 2011
  2. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    This circuit will behave like a non-inverting amplifier with the V+ voltage multiplied by 1+R4/R3.

    The value of V+ is the voltage at the noninverting terminal, which is just (V1 R2+V2 R1)/(R1+R2) which can be easily derived because no current flows into the + terminal.

    Now if all resistors are equal, then the gain is 2 and the V+ is (V1+V2)/2 which means the output is V1+V2.

    So, the derivation is just an extension of the normal noninverting amplifier derivation, which is probably why you had trouble finding it.
     
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  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    Assume: R1=R2=R3=R4 === R

    At amp + in (V+):

    Think of the inputs as a voltage divider between V1 and V2 to get the first term. Then add in V1 as that's what the difference sits on.

    V+ = (V2 - V1) R/(2R) + V1 = (V2 - V1) R/(2R) + V1*2R/(2R)

    = (RV2 - RV1 + 2RV1) / (2R) = (RV2 + RV1) / (2R) = (V2 + V1) / 2

    VOut = V-/R * R + V- = 2V-

    Since V- = V+

    VOut = 2(V2 + V1) / 2 = V2 + V1

    (Sorry, not in the mood to TEX this)
     
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  4. JMac3108

    Active Member

    Aug 16, 2010
    349
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    Whenever you have an op-amp circuit with multiple inputs, the easiest way to solve is usually by superposition. As a reminder, any linear circuit can be solved by superposition. In other words by determining the output from each source individually, then adding up the results. This is done by shorting all the voltage sources except the one you are solving for, then finding the output. Do this for each source, then sum the outputs.

    In this circuit there are two sources, V1 and V2.

    Step 1 - Short V2 to ground and determine output from V1

    The circuit becomes a standard non-inverting amp with a voltage divider consisting of R1 and R2 as the input. The input is V1(R1/(R1+R2)) and this is multiplied by the gain of a non-inverting amp (1+ (R4/R3)).

    Vout1 = V1(R1/(R1+R2)) (1+(R4/R3))

    Step 2 - Short V1 to ground and determine output from V2

    The circuit again becomes a standard non-inverting amp with a voltage divider consisting of R2 and R1 as the input. The input is V2(R2/(R1+R2)) and this is multiplied by the gain of a non-inverting amp (1+ (R4/R3)).

    Vout2 = V2(R2/(R1+R2)) (1+(R4/R3))

    Step 3 - Sum the outputs

    Vout = Vout1 + Vout2

    Vout = V1(R1/(R1+R2)) (1+(R4/R3)) + V2(R2/(R1+R2)) (1+(R4/R3))

    A little simplification yields,

    Vout = [V1(R1/(R1+R2)) + V2(R2/(R1+R2))] (1+(R4/R3))

    A common/typical configuration of this circuit:

    If you made all the resistors the same value, then the equation collapses to the following.

    Vout = [V1(1/2) + V2(1/2)] (2)
    Vout = V1 + V2
     
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  5. Xufyan

    Thread Starter Member

    Aug 3, 2010
    114
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    Thanks alot everyone :)
     
  6. AdrianN

    Active Member

    Apr 27, 2009
    97
    1
    This picture is taken from MasteringElectronicsDesign.com, from the following article: How to Derive the Summing Amplifier Transfer Function. So, the question "How to derive a general equation for a non-inverting summing amplifier" is actually answered in that article. The solution uses the Superposition Theorem.
     
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