Op amp problem please help!

thingmaker3

Joined May 16, 2005
5,083
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.
 

Thread Starter

upopads

Joined Dec 18, 2007
42
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.
 

hgmjr

Joined Jan 28, 2005
9,027
I have succeeded in obtaining the answer that you indicated in your initial post.

That answer is:

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

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
 

Thread Starter

upopads

Joined Dec 18, 2007
42
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.
 

hgmjr

Joined Jan 28, 2005
9,027
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.
Greetings upopads,

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
 

Thread Starter

upopads

Joined Dec 18, 2007
42
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.
 

hgmjr

Joined Jan 28, 2005
9,027
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.
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
 

RmACK

Joined Nov 23, 2007
54
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 :eek:
 
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