# Opamp problem

1. Mar 28, 2016

### Janiceleong26

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

For part ii) ,
V+ is 69mV, but how to determine V-? We don't know the resistance of X, so we can't find the p.d. across X, so how to find V-?
2. Relevant equations
Vout =A0(V+-V-)

2. Mar 28, 2016

### gleem

Isn't the gain of this amp Vout/VR?

3. Mar 28, 2016

### gleem

But Vout and VR are given.

4. Mar 31, 2016

### Janiceleong26

I'm sorry, but can you please explain to me why is V-=0 in your eqn for the gain in post #2 , thanks

Last edited by a moderator: Mar 31, 2016
5. Mar 31, 2016

### gleem

There is no zero in my equation in post #2

Last edited by a moderator: Mar 31, 2016
6. Mar 31, 2016

### Janiceleong26

Then, how did you know that the gain is Vout/VR? Why didn't you take account of V-?

Last edited by a moderator: Mar 31, 2016
7. Mar 31, 2016

### gleem

That is the general definition of gain.

8. Mar 31, 2016

### Janiceleong26

OK.
Vout=Ao(V+-V-), am I right?
Vout=5.8V
V+=69mV
V-= ?

9. Mar 31, 2016

### gleem

Why do you want V- ?

10. Mar 31, 2016

### Janiceleong26

So that I can use the formula, Vout=A0(V+-V-) ,and get A0

11. Mar 31, 2016

### gleem

The question was what is the gain of the circuit not the op-amp. that's why I commented on the simplicity of the question.

12. Mar 31, 2016

### LvW

So you are interested in Ao ???
Up to now I was of the opinion you were looking for the gain of the whole amplifier stage?

13. Mar 31, 2016

### Staff: Mentor

[ Mentor note: this thread has been tidied up ]

14. Mar 31, 2016

### Janiceleong26

Ohhh.. I get it now
Sorry I thought they were asking for A0
Thanks so much