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Operational amplifier circuit

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paulmdrdo

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In the problem below, when I used node V1 and V2 I get the correct answer of Vo = -2.5v
But when I use nodes V1 and Vo I get an incorrect answer. Please explain to me why my latter method did not work.
Thanks,

Paul
sadiku p.5.14.png
 

Hi
What you are missing is the current in/out of the OpAmp. In your equation for Vo you only say there is current going to V1 and V2, but there is also a contribution going to the OpAmp.

Regards
 
Your second equation is V1 + 2Vo = 0 and your fourth equation is -V1 + 2Vo = 0

These can't both be true unless V1 = 0, which is not true.

But your first result is also not true. Vo is not -2.5 volts; it's -2.857 volts.

If you would show how you derived your equations, we can help you find your mistake.
 

It's not -2.857; it's 2.5. At least according to my simulation with LTSPICE. My calculation also gives 3.3, but I'm not sure where I messed up. Assumed V2 is zero volts (virtual ground).
 

The correct answer is Vo=-2.5V as can be seen, simply, applying the superposition.

I don't understand how you derive the wrong equation: -V1+2Vo=0

The correct equation is V1+2Vo=0 or -V1-2Vo=0
 

Checking my variables, I see that I must have gotten a key bounce on one of them when I typed in all those values with so many zeroes. :-(

Vo is indeed -2.5 volts. But we still don't know how the OP got that last equation, so we can't help him.
 

Hi,
To derive this equations paulmdrdo applied correctly Nodal analysis at node labeled V1 and V2 to get Vo=-2.5V, and incorrectly at nodes labeled V1 and Vo (as poined out by tyassin).
The correct set of ecuations are:

1.Applied at node V1 and V2:
2mA= V1/5k+(V1-Vo)/10k+V1/20k
V1/20k+Vo/10k=0

2.Applied at node V1 and Vo:
2mA= V1/5k+(V1-Vo)/10k+V1/20k
(V1-Vo)/20k + Iamp_out = Vo/20k

Obviously you cand find Vo form the first set of ecuations. In the second set, you need to know what Iamp_out is.
 
Shouldn't the last equation be (V1-Vo)/10k + Iamp_out = Vo/10k?

It seems clear that this is what paulmdrdo did. He wrote that last equation and ignored Iamp_out.
 

paulmdrdo said:
In the problem below, when I used node V1 and V2 I get the correct answer of Vo = -2.5v
But when I use nodes V1 and Vo I get an incorrect answer. Please explain to me why my latter method did not work.
Thanks,

Paul
View attachment 145725
Click to expand...

Problem has 3 equations with 3 unknows.

paulmdrdo.JPG

Ratch
 

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