OP-AMP subtraction circuit (how was vo derived in this circuit?)

Discussion in 'Homework Help' started by electrogirl, Aug 12, 2010.

May 2, 2010
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I already tried solving this but so far can only solved when there is only one input that is given but this time there are two inputs?How was Vo arrived in this circuit?(see attachment for the figure) thanks!

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2. hgmjr Moderator

Jan 28, 2005
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Can you post the results of your efforts to solve the problem?

hgmjr

electrogirl likes this.

May 2, 2010
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this is how I analyze this circuit...is this correct?What should I do next?

• subtraction circuit1.png
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May 11, 2010
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electrogirl, that is very interesting to do it by currents. however, i don't see why you marked v1 and v2 in different places from the v1 and v2 at the input. that makes it very hard because they are different voltages with the same label.

Here is how I did it. I wrote the equation for the two inputs of the op amp in terms of voltage, and set them equal.

Then I solved for Vo and I get the result that they show.

So...

V1 ( R3/(R1+R3) ) == V2 + (Vo - V2)( R2/(R2+R4) )

and from there, solve for Vo. It's not so easy, for me at least, but after some time i got it.

electrogirl likes this.

May 2, 2010
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I wrote V1 and V2 in those nodes because the voltages in those nodes are equal to V1 and V2 respectively.
why did you set the two inputs equal?

May 11, 2010
241
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There is already a V1 and V2 in the drawing, and the voltages are not equal to the inputs of the op amp. The voltages are created by the dividers between V2 and V0, and V1 and Ground.

I set the two input voltages to the op amp equal because that is the way that the op amp works. As long as you have the feedback in the correct direction, it will try to maintain the input voltages equal.

electrogirl likes this.