# Op-Amp output calculation

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

1. ### StuckStudent123 Thread Starter New Member

Sep 23, 2016
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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 AAC Fanatic!

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

Sep 23, 2016
12
0

Jun 4, 2014
6,364
1,475

5. ### StuckStudent123 Thread Starter New Member

Sep 23, 2016
12
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Third time lucky!

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6. ### AlbertHall AAC Fanatic!

Jun 4, 2014
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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.

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7. ### StuckStudent123 Thread Starter New Member

Sep 23, 2016
12
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So do you not have to do anything with V1, the non-inverting?

8. ### AlbertHall AAC Fanatic!

Jun 4, 2014
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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
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That it's almost 0?

10. ### AlbertHall AAC Fanatic!

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

11. ### StuckStudent123 Thread Starter New Member

Sep 23, 2016
12
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They're the same?

12. ### AlbertHall AAC Fanatic!

Jun 4, 2014
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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
18,078
9,617
Any op-amp running properly in a closed loop gain scenario will null its inputs.

14. ### AlbertHall AAC Fanatic!

Jun 4, 2014
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Important result to remember.

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