Is Vb 0V in this Op-Amp circuit?

[Cocoleia]

Homework Statement

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

Homework Equations

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?

 

[gneill]

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.

 

[Cocoleia]

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.

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

 

[gneill]

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

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).

 

[Cocoleia]

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).

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

 

[gneill]

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

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

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

 

[Cocoleia]

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

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

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

 

[gneill]

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

 

[Cocoleia]

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

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

 

[gneill]

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

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