Discussion in 'Homework Help' started by upopads, Mar 30, 2008.

Dec 18, 2007
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2. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
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This is not a "homework for pay" site. We'll help with advice and hints, and we'll confirm correct answers. But we will not do your work for you.

First find Vo. Then what you have is an algebra problem.

3. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Here's the circuit - clicking on the link seemed to take forever to come up!

Dec 18, 2007
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Is Vo just the inverting op amp equation (-Rf/R1)*Vs?

Dec 18, 2007
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No matter what I do I can't get Rf to cancel out correctly.

Dec 18, 2007
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I've been working on this for a while on paper and I've done it a numerous amount of incorrect ways. The answer simplifies to Vj = (R1-KRj)Vs.

7. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
I contest that answer. If Rj approaches zero, then Vj will approach zero as well, and (R1-KRj)Vs will approach (R1)Vs.

8. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
No. There is feedback on the + input as well as the - input.

Remember: an op amp will produce an output as needed to bring the two inputs to the same voltage (if at all possible).

9. ### hgmjr Moderator

Jan 28, 2005
9,030
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I have succeeded in obtaining the answer that you indicated in your initial post.

$V_{}j =V_{S}\left(\frac{1}{1-\frac{R_{1}}{K*R_{j}}}\right)$

The steps I used were

1. Write the expression for the voltage at the opamp's negative terminal as a function of Vs, R1, Rf, and Vout.

2. Write the expression for the voltage at the opamp's positive terminal as a function of Rf/K, Rj, and Vout.

3 Set the expression from step 1 equal to the espression from step 2 and solve for Vout.

4. Plug the expression for Vout from step 3 into the expression for the voltage at the opamp's positive terminal from step 2 and then solve.

hgmjr

Dec 18, 2007
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That equation is written correctly, except for that Vout is supposed to be Vj. I'm still not quite getting it, i always end up with Rf at the end.

11. ### hgmjr Moderator

Jan 28, 2005
9,030
214

Thanks for alerting me to the error in my equation.

If you can post the results of your attempt at step 1 and step 2 as described in my earlier reply, someone will help you with any difficulty you are having with getting those expressions correct.

hgmjr

Dec 18, 2007
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Well I've been writing the first equation as Vn=-Vs(Rf/R1) and the second equation as a parallel combination of the two resistors Rj and Rf/K times Vo or simplified Vp=Vo([Rf*Rj]/[Rf+Rj*K]) and then I set Vp=Vn and solve for Vo but i can't get rid of the Rf term when i do that.

13. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Unfortunately, the expressions you are using for Vn and Vp are not yet correct.

I would recommend that you focus on the expression for Vp for the moment and temporarily replace the resistance Rf/K with say a resistor Rx. This just keeps things simpler until you have determined the correct expression.

hgmjr

14. ### Distort10n Active Member

Dec 25, 2006
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The signal on the non-inverting pin is not feedback. The resistor RfK isolates the non-inverting input from the output of the op-amp.

15. ### RmACK Active Member

Nov 23, 2007
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Assuming the negative feedback is working correctly etc, we'll assume that V-=V+=Vj the inputs of the opamp are equal. -will check later.
Then Vj=VoRj/(Rf/K+Rj) or Vo=Vj((Rf/(KRj))+1)
(Vs-Vj)/R1=(Vj-Vo)/Rf
We want Vj in terms of Vs so remove that Vo with equation given earlier:
(Vs-Vj)/R1=(Vj-Vj((Rf/(KRj))+1)
If this is correct, it should rearrange but that is not something I would like to try to do

16. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
That's just plain wrong.