# Non Inverting summing amplifier

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

1. ### naspek Thread Starter Member

Feb 17, 2010
45
0
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
derivation...

i know that
Vo = 1/n (V1 + V2 ... +Vn)(Rf/R1 + 1)
but.. how to get it step by step? apply KCL? VDR?

Last edited: Mar 18, 2010
2. ### rogs Active Member

Aug 28, 2009
279
37
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
45
0
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?