# Summing amplifier (how to set it up)

Discussion in 'General Electronics Chat' started by Cretin, Oct 12, 2013.

1. ### Cretin Thread Starter Member

Dec 13, 2012
68
1
Hey all I'm having a couple of problems designing a summing amplifier, and hoping you can help me out.

So i am trying to design a summing amplifier as found below

My requirements are:

- when my R3 (potentiometer) is adjusted from one end of its travel ot the other, the Vo should be 4.0V p-p sinewave that has a DC offset, that varies from 5.0 to 10.0 volts

I'm having a hard time calculating the values of R1,R2 and Rf, these are the formulas I have but I just can't get the values right...

Here are my given formulas

For 5VDC 1/3 = Rf/Ri+1000 and Rf/Ri = 4

for 10VDC 2/3 = Rf/R2

I am not quite sure what to do with these values. I havesolved for Rf and Ri so that

Rf = 3500
R1 = 875
R2 = 5200

Are those correct according to the calculations?

2. ### crutschow Expert

Mar 14, 2008
16,206
4,330
According to my calculations the correct values are:
Rf = 6.67kΩ
R1 = 1.67kΩ
R2 = 10kΩ

I leave their derivation as an exercise for the reader.

Cretin and anhnha like this.
3. ### LvW Active Member

Jun 13, 2013
674
100
Cretin, I am afraid you have to recalculate.
Example: R3=10k

Actual Gain(DC)=-RF/(R2+R3)=-3.5/(15.2)=-0.23
Required: -5/15=-1/3=-0.3333

4. ### Cretin Thread Starter Member

Dec 13, 2012
68
1
Hey guys,

Thanks for your help first off crutschow, i see now that to calculate the gain of each line coming into the inverting pin, i have to do a gain calculation for both R2+R3 and R1