gain of inverting op amp?

Discussion in 'Homework Help' started by no12thward, Oct 25, 2011.

  1. no12thward

    Thread Starter Member

    Jan 10, 2011
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    I'm looking for two resistor values from the table that will give me a gain of -18. Ive tried converting to kilo, milli, micro, nano ohms, etc, then using -(Rf/Ri). Cant seem to find any combo of resistor values on the table that will give me the gain of -18.
     
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    There are two number combinations from your table that will work ... how do you think you should go about and solve this problem?
     
  3. no12thward

    Thread Starter Member

    Jan 10, 2011
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    i know I'd have to change one of the resistor values from Ωs to a larger or smaller ohm conversion, but I've tried that for R1 as well as R2 resistors. Maybe im missing something?
     
  4. Georacer

    Moderator

    Nov 25, 2009
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    Just look at your values more carefully. You have available all of the powers of 10 of these values. For example, you can use 1Ω, 10Ω, 100Ω etc.

    Does that make it any easier?

    These papers you post lately aren't the most strict and best-written exercises, granted.
     
  5. no12thward

    Thread Starter Member

    Jan 10, 2011
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    Sorry, it doesnt. im not seeing any power of ohm value i can use to get a gain as low as 18. I've used all the conversions
     
  6. no12thward

    Thread Starter Member

    Jan 10, 2011
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    how can i derive anything above 9.1ohm (10ohm or 100ohm) from the values on the table?
     
  7. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    9.1, 910, 9100, 91000, 910000, 9100000

    Keep multiplying by 10.

    There are two groups that satisfy your requirements.
     
  8. Georacer

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    Nov 25, 2009
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    Another hit:

    If I wanted a gain of -11, I could use a resistor of 11kΩ and a resistor of 1kΩ.
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    Your list is missing the standard resistance value of 1.8, 18, 180, 1.8k 18k and 180k.
    A 1k input resistor and 18k feedback resistor or a 10k input resistor and a 180k feedback resistor results in a gain of 18 times.
     
  10. steveb

    Senior Member

    Jul 3, 2008
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    I think it is not just missing, but instead it looks like there is a typo where the 1.7 should be 1.8.
     
  11. Georacer

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    Nov 25, 2009
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    Since this is homework, let's assume that the teacher has set the restrictions that he feels they will best test the student's knowledge.

    It's rare that homework that eludes the restrictions but applies to everyday rules gets rewarded.
     
  12. no12thward

    Thread Starter Member

    Jan 10, 2011
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    I'm thinking I could use Rf=2 and Rin=1.1. But then how would I convert the ohm values to get 18?
     
  13. Georacer

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    Nov 25, 2009
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    The closest possible gain you could achieve with those coefficient would be
    A=-\frac{2*10^1}{1.1}=-18.18

    It is close, but you can do better.

    Remember that if |A|=\frac{R_f}{R_i} then R_f=|A| \cdot R_i
    You can input various values for R_i and see if you have the R_f available.
     
  14. no12thward

    Thread Starter Member

    Jan 10, 2011
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    I think were allowed a 1% error. My only question would be, with only being able to use these standard resistor values...if I use the 2ohm, the next size up would be a milliohm right? How could I use 2 X 10^1. Wouldn't that exceed my resistor values from the table?
     
  15. Georacer

    Moderator

    Nov 25, 2009
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    Your don't need to insert error if you can help it.

    A mili is equal to 10^{-3}, so in that aspect you are wrong.

    Pay a bit more attention. It has been stated numerous times that the exercise wants you to use any power of 10 you want to multiply your values with. It cannot be done any other way.

    I 'll give you another hint: You can set R_i=1.5K\Omega. How much will R_f be, according to the formula I gave you previously?
     
  16. no12thward

    Thread Starter Member

    Jan 10, 2011
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    Would be 2.7?
     
  17. Georacer

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    Nov 25, 2009
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    Please formulate complete answers.

    2.7what? If I assume that you mean 2.7Ω then the gain will be A=-\frac{2.7 \Omega}{1.5 \cdot 10^3 \Omega}=-0.0018 which is clearly wrong.

    Try once more.
     
  18. no12thward

    Thread Starter Member

    Jan 10, 2011
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    Sorry. 2.7X10^1Ω
     
  19. Georacer

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    Nov 25, 2009
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    I gave you R_i=1.5K\Omega, not R_i=1.5\Omega.

    With R_f=27\Omega you will have a gain of A=-\frac{27}{1500}=-0.018, once again not acceptable.
     
  20. SgtWookie

    Expert

    Jul 17, 2007
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    I think the 1.7 is a deliberate error to keep students from jumping at it.
     
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