# Homework Help: What is Rf -> Op_Amp

1. Oct 14, 2012

### Metamorphose

What is Rf --> Op_Amp

1. Consider the amplifier circuit shown. What value of Rf will yield vout = 2V when Is = 10 mA and Ry = 2Rx = 500Ω

2. The way I did this was by employing KCL: (Is that applicable?)

3. The attempt at a solution

Rx(Is) + Rf(Is) = Vout

I'm actually not quite sure how to do this one. It's left me in a bit of a pickle.

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2. Oct 14, 2012

### Staff: Mentor

Re: What is Rf --> Op_Amp

I don't see how it's possible. The potential at the op-amp + input will be negative, and feedback should drive the - input to the same value. Thus Vout will be less than that (so long as only positive resistance values are permitted).

3. Oct 15, 2012

### Staff: Mentor

Re: What is Rf --> Op_Amp

Perhaps someone has introduced a typo—and that output of 2V should actually be shown as –12V?

4. Oct 15, 2012

### CWatters

Re: What is Rf --> Op_Amp

As gneil said...

Should that be Vout = -2V or Is = -10mA ?

5. Oct 17, 2012

### rude man

Re: What is Rf --> Op_Amp

R_y is obviously a negative dc resistance!

6. Oct 17, 2012

### rude man

Re: What is Rf --> Op_Amp

No combination of + or - 10mA and + or - 2V is possible.

7. Oct 17, 2012

### CWatters

Re: What is Rf --> Op_Amp

Humm, you're right.

If Is were -10mA then V+=+5v. For the amp to be in a linear mode V- ≈ V+. However V- can't be +5V if Vo is only 2V.

If Is were -10mA and Vo was -2V you have same problem. V- can't be +5V if Vo=-2V.

8. Oct 17, 2012

### rude man

Re: What is Rf --> Op_Amp

Yup. Point is, in the noninverting mode, the gain can't be less than 1.

Unless you have active feedback (a second op amp configuration feeding the output of the first op amp back to its input.)