# Op Amp KCL loop question

Discussion in 'Homework Help' started by Nathen Leung, Feb 20, 2016.

1. ### Nathen Leung Thread Starter New Member

Feb 20, 2016
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I asked the professor and he hinted that I should use a KCL loop at the inputs with virtual ground. But I still don't fully understand it myself.

2. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Is that negative resistance?

3. ### Nathen Leung Thread Starter New Member

Feb 20, 2016
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No it's just a dash. Values are 91k and 51k

4. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Ok.
Can you follow the current from V1 and to the ground? Call this current A. Can you write out which components it goes through? What the voltage would be across that component?

Feb 20, 2016
7
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Yep
Yep

6. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Write it out. I want to see an equation.

7. ### Nathen Leung Thread Starter New Member

Feb 20, 2016
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Would current A be going through R1 and R2. I don't know what the voltage that would be

8. ### shteii01 AAC Fanatic!

Feb 19, 2010
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482
Almost correct. It will go through R1, R2 and Rgen1.

So you are looking at something like this:
A*R1+A*R2+A*Rgen1-V1=0
A*R1+A*R2+A*Rgen1=V1
A*(R1+R2+Rgen1)=V1
A*(91+91+51)=V1
233A=V1

Now do the same for the V2. Some current is leaving V2, call it current B, follow its path. Also you might want to redraw the "V2 circuit". Instead of Vout being a dot in space, replace it with a circle like the other independent voltage sources.

9. ### Nathen Leung Thread Starter New Member

Feb 20, 2016
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So then BR4+BR3+BRgen2=V2 and since its a difference amplifier do I subtract the voltages?

10. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Vout is connected to R4... It is part of this circuit.

11. ### Nathen Leung Thread Starter New Member

Feb 20, 2016
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Oh woops! so BR4+BR3+BRgen2+V2=vout

12. ### WBahn Moderator

Mar 31, 2012
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What assumptions are you making about the op-amp? Are you assuming it is ideal? Even though it is stated that it's a 741?

Part (b) talks about "both circuits" -- what "both"?

13. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,294
482
That is good. So.
Vout=B*R4+B*R3+B*Rgen2+V2
Vout=B*(R4+R3+Rgen2)+V2
Vout=B*(91+91+51)+V2
Vout=233B+V2

Now. Tell me about the "virtual ground" junctions. What do we know about those junctions? The junctions that I am talking about are:
- Junctions of R1 and R2 and OA1 positive input
- Junction of R3 and R4 and OA1 negative input
What do we know about these junctions?

14. ### Nathen Leung Thread Starter New Member

Feb 20, 2016
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0
Well it's asking for vout in term of V1 and V2 and I'm assuming that its a difference amplifier. I'm guessing there was 2 circuits to the problem but he removed one

15. ### WBahn Moderator

Mar 31, 2012
17,434
4,694
But what you get for Vout in terms of V1 and V2 depends on what you assume about the opamp. In particular for this circuit, are you taking into account the input bias current or not?