Hi,
Is anyone able to help me with this op amp calculation?
It's the 1k resistor that's confusing me a bit. If it wasn't there I'd imagine it would just be a simple differential amplifier (but I might be wrong there too).
Rx=2k, V1=3v, V2=9v and the question asks for V0.
Thanks

 

I can't open the picture.

 

https://goo.gl/8vQB5o

 

That one requires me to log in.

 

Third time lucky!

 

OK!
What is the voltage at the inverting input? Hint: No calculation required.
Given that, you know the currents in Rx and the 1k resistors and therefore the current in the 2k resistor. Now you can calculate Vo.

 

So do you not have to do anything with V1, the non-inverting?

 

You most certainly do!
The output voltage is the difference between the input voltages multiplied by the gain of the amplifier. If we are taking the gain of the amp as infinite, what does this tell you about the voltage difference between the inputs?

 

That it's almost 0?

 

Yep, and you know the voltage at the non-inverting input so...

 

They're the same?

 

Yep.
Now you know the voltage at the non-inverting input, you can calculate the current through Rx and R1
And what is the current into the non-inverting input of the amp?
And therefore what is the current in R2?

 

They're the same?

Any op-amp running properly in a closed loop gain scenario will null its inputs.

 

Any op-amp running properly in a closed loop gain scenario will null its inputs.

Important result to remember.