# Opamp problem

1. Homework Statement

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-?

Vout =A0(V+-V-)

## Answers and Replies

gleem
Science Advisor
Education Advisor
Isn't the gain of this amp Vout/VR?

CWatters
gleem
Science Advisor
Education Advisor
But Vout and VR are given.

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

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gleem
Science Advisor
Education Advisor
There is no zero in my equation in post #2

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There is no zero in my equation in post #2
Then, how did you know that the gain is Vout/VR? Why didn't you take account of V-?

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gleem
Science Advisor
Education Advisor
That is the general definition of gain.

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

gleem
Science Advisor
Education Advisor
Why do you want V- ?

Why do you want V- ?
So that I can use the formula, Vout=A0(V+-V-) ,and get A0

gleem
Science Advisor
Education Advisor
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.

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

NascentOxygen
Staff Emeritus
Science Advisor

[ Mentor note: this thread has been tidied up ]

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