# Op-Amp output calculation

Discussion in 'Homework Help' started by StuckStudent123, Sep 23, 2016.

1. ### StuckStudent123 Thread Starter New Member

Sep 23, 2016
10
0
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

2. ### AlbertHall Well-Known Member

Jun 4, 2014
2,270
448
I can't open the picture.

Sep 23, 2016
10
0

Jun 4, 2014
2,270
448

5. ### StuckStudent123 Thread Starter New Member

Sep 23, 2016
10
0
Third time lucky!

File size:
12.9 KB
Views:
16
6. ### AlbertHall Well-Known Member

Jun 4, 2014
2,270
448
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.

StuckStudent123 likes this.
7. ### StuckStudent123 Thread Starter New Member

Sep 23, 2016
10
0
So do you not have to do anything with V1, the non-inverting?

8. ### AlbertHall Well-Known Member

Jun 4, 2014
2,270
448
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?

9. ### StuckStudent123 Thread Starter New Member

Sep 23, 2016
10
0
That it's almost 0?

10. ### AlbertHall Well-Known Member

Jun 4, 2014
2,270
448
Yep, and you know the voltage at the non-inverting input so...

11. ### StuckStudent123 Thread Starter New Member

Sep 23, 2016
10
0
They're the same?

12. ### AlbertHall Well-Known Member

Jun 4, 2014
2,270
448
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?

13. ### #12 Expert

Nov 30, 2010
16,703
7,341
Any op-amp running properly in a closed loop gain scenario will null its inputs.

14. ### AlbertHall Well-Known Member

Jun 4, 2014
2,270
448
Important result to remember.

#12 likes this.