Can someone help get started with this...

Discussion in 'Homework Help' started by ///m3dave, Jun 16, 2010.

  1. ///m3dave

    Thread Starter New Member

    Dec 8, 2009
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    [​IMG][​IMG]a. Determine the value of R1 for the OP Amp
    circuit shown. AV(dB) = 33dB.
    [​IMG]
    Thanks
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    What ideas do you have to solve the problem?

    hgmjr
     
  3. ///m3dave

    Thread Starter New Member

    Dec 8, 2009
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    I know that the answer is 223 ohm. But I can't figure out how to get that. From what I've been reading in my book the only thing I can come up with is to divide 33dB by 10 to get Vo, then multiply that by 10k ohm?
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    The factor of 10 is normally associated with power gain. The problem statement here appears to be requesting voltage gain. In that case you would use 20 rather than 10.

    Go to www.allaboutcircuits.com or your textbook and study the information on decibels.

    The first step that I would suggest is that you convert the 33db voltage gain into the Voltage-out/voltage-in.

    hgmjr
     
  5. Georacer

    Moderator

    Nov 25, 2009
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    And make sure you know exactly wich is the gain of the inverting assembly.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    It is an impractical circuit because its input resistance is very low because the resistor values are very low. Its voltage gain is 33dB only when it is fed from an extremely low impedance.
     
  7. Ghar

    Active Member

    Mar 8, 2010
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    So then it's practical for a low impedance source and useful otherwise if you don't need the full 33 dB... what's the issue?
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    If the opamp is non-inverting then it could have a high or very high input impedance and still have plenty of gain.
     
  9. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Impractical or not ... this is some professors idea of an analysis. If we argue the practicality of the circuit, the reason for the problem is lost, which doesn't fulfill the OPs inquiry.

    Your issue is not with the OP but their professor and I'm sure the OP can message you with the Professor's email address so you can take the practicality discussion offline or invite the professor to a new thread discussing the practicality of the professor's assignment.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    If the professor insists on studying an inverting opamp then maybe its feedback resistor should be 1M instead of only 10k.
     
  11. Ghar

    Active Member

    Mar 8, 2010
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    What if you're building a low noise circuit and there is a lot of unavoidable current noise from some devices?
    There's plenty of reason not to use huge resistors if you don't have to.
     
  12. tskaggs

    New Member

    Jun 17, 2010
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  13. tskaggs

    New Member

    Jun 17, 2010
    26
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    Nvm, just change the 10 in my solution to 10k.
     
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