# Summing/Differential? Amplifier

Discussion in 'General Electronics Chat' started by Axel, Nov 2, 2008.

1. ### Axel Thread Starter New Member

Nov 2, 2008
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0
I have a question that OpAmp looks like Summing but there are Voltage applied to the + input.

note that there are 2 input voltage 150mV and -153mV to the 741 OpAmp, i know the calculation of Summing Amplifier and the value shud be 0.28V, but when connect a 0.5V to the + input, it bcome looks like differential but without Rf and Ri, and the output become 3.188V, can someone show me the formula for getting 3.188V?

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Last edited: Nov 2, 2008
2. ### hgmjr Moderator

Jan 28, 2005
9,030
214
The gain of the opamp for signals applied to the + input is 5.8. When you mulitply 0.5 times 5.8 you get 2.9V. The balance of the output signal is accounted for by the other two voltage inputs and their associated gains.

In summary, the simulated opamp is operating correctly.

hgmjr

3. ### mik3 Senior Member

Feb 4, 2008
4,846
63
If you make the analysis of the circuit you get this equation:

Vo=(-V1+0.5)*R5/R6+(-V2+0.5)*R5/R3+0.5

Plug the numbers in and you get the answer the simulation gave you.

4. ### hgmjr Moderator

Jan 28, 2005
9,030
214
The above equation is the same as mik3 using the LaTex format.

hgmjr