What is Rf for Op-Amp Circuit w/ Vout=2V & Is=10mA?

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In summary, the problem presents a circuit with an unknown value of Rf, and asks for the value that will result in a vout of 2V when the current Is is 10 mA and the resistance values of Ry and Rx are given. The conversation discusses the issue of whether the given values are possible, and concludes that they are not due to the limitations of the noninverting amplifier mode.
  • #1
Metamorphose
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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?)



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


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

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


Perhaps someone has introduced a typo—and that output of 2V should actually be shown as –12V? :smile:
 
  • #4


As gneil said...

Should that be Vout = -2V or Is = -10mA ?
 
  • #5


R_y is obviously a negative dc resistance! :bugeye: :eek: :frown:
 
  • #6


CWatters said:
As gneil said...

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

No combination of + or - 10mA and + or - 2V is possible.
 
  • #7


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


CWatters said:
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.

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.)
 

1. What is Rf in an Op-Amp circuit?

Rf stands for feedback resistance and it is the resistor that is placed in the feedback loop of an Op-Amp circuit.

2. What is the purpose of Rf in an Op-Amp circuit?

The purpose of Rf is to provide negative feedback to the Op-Amp, which helps to stabilize and control the output voltage.

3. How does Rf affect the output voltage in an Op-Amp circuit?

Rf is directly proportional to the output voltage of an Op-Amp circuit. This means that as Rf increases, the output voltage also increases.

4. What is the recommended value for Rf in an Op-Amp circuit with Vout=2V and Is=10mA?

The recommended value for Rf can vary depending on the specific Op-Amp and circuit design, but a good starting point would be to choose a value that is at least 10 times larger than the input resistor (Is). In this case, a value of 100kΩ would be a good starting point.

5. Can Rf be a variable resistor in an Op-Amp circuit?

Yes, Rf can be a variable resistor in an Op-Amp circuit, which allows for adjustable output voltages. However, it is important to choose a variable resistor with a high enough range to cover the desired output voltage range.

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