# Node voltage with opamp

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1. Jan 23, 2017

### Cocoleia

1. The problem statement, all variables and given/known data

I am wondering if in this circuit Vb will be 0V?

2. Relevant equations

3. The attempt at a solution
I have found all the other ones, but I am unsure if Vb will be 0 or not, and if it is then what is the logic behind it?

2. Jan 23, 2017

### Staff: Mentor

If you've found all the others then you must know the voltage at node c. That means you have everything you need to do nodal analysis for node b.

3. Jan 23, 2017

### Cocoleia

Not quite, I am given the answers for each node and asked to prove that they are right, all of my proof uses the fact that Vb=0, but I don't know how to prove that

4. Jan 23, 2017

### Staff: Mentor

Then you need to analyze the circuit, starting by using the properties of the ideal op-amp. Start by finding the potential at its + input (node f).

5. Jan 23, 2017

### Cocoleia

I wrote all the equations for nodes b, c, d and f. Since we can see that Va=-5V and Ve=2v, from the drawing. I just end up with 3 eqns and 4 unknowns

6. Jan 23, 2017

### Staff: Mentor

You'll have to show the details if you want help.

You should be able to determine the potential at node f by inspection.

7. Jan 23, 2017

### Cocoleia

But here I take the assumption that Vb=0, I want to know if that is logical or if it would not be 0V

8. Jan 23, 2017

### Staff: Mentor

You know the potential at f, so using the properties of the op-amp what must be the potential at its "-" input?

9. Jan 23, 2017

### Cocoleia

Ok so potential at f and c have to be the same ?

10. Jan 23, 2017

### Staff: Mentor

Yes. So long as an op-amp circuit provides a negative feedback path, it must be so.