Find the cut-off frequency

Discussion in 'Homework Help' started by basim, May 12, 2009.

  1. basim

    Thread Starter New Member

    May 12, 2009
    1
    0
    the question is
    From you result in table Find the cut-off frequency


    circuit figure
    [​IMG]


    lab result

    [​IMG]
    Images Like This One​



    thanks so much in advance
     
  2. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    A cutoff frequency is often defined as the point where the gain of the filter/amplifier is down 3 dB. Is that how it is defined in your class?

    Are there any points in your table that meet that criterion?
     
  3. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    I sure don't see a cutoff condition on that table. Are you sure you're asking the right question?
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    If those are lab results, you should have started your input at a lower frequency, and ended at a higher frequency.
     
  5. BrianH

    Active Member

    Mar 21, 2007
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    0
    Hi Ron.H;

    Since there are no bandwidth limiting components placed in that circuit, surely the cut-off will not occur until the input frequency is increased high enough to start the miller effect?

    So, with that in mind, I will agree that he should have taken measurements at a much higher frequency but I'd suggest 20Hz was a more than adequate starting point?

    Brian.
     
  6. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    A capacitor is typically placed between the source and the input to remove the DC component of the source. Same goes for the output. So, looking at the circuit, there is reason to believe that there is something going to happen at the low end of the spectrum. The data also begins to show the falling off of the gain.

    I would suggest starting your input at a lower frequency and ending at a higher frequency as Ron H suggested.
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Well, the emitter bybass cap and resistors should set the low end corner at around 20Hz, but his data shows that the response was almost flat at that frequency. The results imply that the cap value is closer to 250uF, rather than 100uF.
    As for the high end, if the source impedance were really zero, Miller capacitance would not be a factor. A real-world generator is likely to have 50Ω or 600Ω output resistance, so the Miller effect will be present. In simulation, 500Ω matches his midband gain pretty closely. A 10pF scope probe will lower the high end corner frequency considerably.
     
  8. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    Basim, you obviously messed up, but you need to correct it -- for your own sake. There should be an hour that gives you access to a scope at school. Here's something I found out only in my last year at the university: The physics department would let you have anything you needed, like checking out books at the library. I checked out a set of weights (accurate grams scale type), but I realized I could have had just about anything I wanted right there in my dorm room!
     
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