Operational amplifier circuit

[paulmdrdo]

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

 

[tyassin]

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

 

[The Electrician]

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.

 

[barry]

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).

 

[albbg]

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

 

[The Electrician]

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.

 

[ZoOneR]

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.

 

[The Electrician]

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.

 

[Ratch]

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

Problem has 3 equations with 3 unknows.



Ratch