Collector emmiter open circuit voltage

Discussion in 'Homework Help' started by ham3388, Oct 26, 2016.

  1. ham3388

    Thread Starter Member

    Jul 3, 2012
    97
    3
    Hello dear friends
    Please go through my work. I don't know how to find the value of CEO voltage.
    I have done the remaining.
    Could someone

    20161026_223253.jpg

    20161026_223436.jpg

    20161026_223343.jpg
     
  2. crutschow

    Expert

    Mar 14, 2008
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    I see some errors in your calculations starting with the second equation.
    It can't be 24V as you are not including any voltage drop across the transistor.

    First you calculate the approximate voltage at the base due to R3 and R4 (initially ignoring the small base current), and then go from there to calculate the emitter and collector voltages and current.
    After that you can backtrack to include the small base current voltage drop due to the collector current in the equivalent R3||R4 resistance and tweak the base voltage.
    Then go back and tweak the resulting emitter and collector voltages.

    Here's the results of my LTspice simulation:
    See if you can get your calculations to be reasonably close to those values.
    (The simulation has about a Vbe of 0.7V rather than the 0.6V you are instructed to use, so that's one difference).

    upload_2016-10-26_12-52-32.png
     
    Last edited: Oct 26, 2016
  3. MrAl

    AAC Fanatic!

    Jun 17, 2014
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  4. crutschow

    Expert

    Mar 14, 2008
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    He can't do that until he properly solves for the transistor voltages.
     
  5. anhnha

    Well-Known Member

    Apr 19, 2012
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    I see many steps here and not sure how you did this. Should it be easier to use Thevenin equivalent right from beginning?
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Whatever floats your boat. :D
     
  7. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hi,

    Hasnt he done that already?

    What i was asking is based on what he already calculated, what is his choice for the transistor, as that was the whole point of doing this in the first place. He has three choices and he has to pick one now.

    My boat sank a long time ago :)
     
  8. Ramussons

    Well-Known Member

    May 3, 2013
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    Since no pricing has been mentioned, irrespective of the voltages calculated, I'd choose BC 107. It has the best ratings among the 3 choices.
     
  9. ham3388

    Thread Starter Member

    Jul 3, 2012
    97
    3
    Hi again
    Keeping your advices in mind, I recalculated them again.
    If the values below are correct. , please show me how to calculate Vceo.

    Here is my work

    20161030_163823.jpg
     
  10. ham3388

    Thread Starter Member

    Jul 3, 2012
    97
    3
    I found error in the calculation
    Here is the corrected one

    20161030_165353.jpg
     
  11. crutschow

    Expert

    Mar 14, 2008
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    I believe your last calculations are correct. :)
    The difference between my simulation and your calculations is that you ignored the effect of the base current on the base voltage from the input bias resistors.
     
  12. ham3388

    Thread Starter Member

    Jul 3, 2012
    97
    3
    So how to find the Vceo ?
     
  13. ham3388

    Thread Starter Member

    Jul 3, 2012
    97
    3
    Does it mean Vce when the base is open circuit?
     
  14. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    You read the datasheet.
     
  15. ham3388

    Thread Starter Member

    Jul 3, 2012
    97
    3
    I need to calculate it and then compare it with the value in the data sheet. , but how?
     
  16. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    What you have found is Uceq ( quiescence point, operating point of a device) ≈ 10.7V and Ptotq ≈ 0.22W. And now, based on those calculations you have to choose (select) a proper (suitable) transistor from table B.

    And  V_{CE0} is the maximum allowable voltage that can be applied from the collector to the emitter (with base terminal left open) of a transistor before it becomes damaged (before magic smoke escape) .
     
  17. crutschow

    Expert

    Mar 14, 2008
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    Yes.
    Vceo is the worst-case condition for the maximum collector emitter voltage so you would look for a transistor that has a Vceo rating of at least 50% more than the maximum Vce the transistor would see in the circuit for good reliability.

    Note that this maximum voltage with an AC input will be greater than the bias value you calculated.
     
  18. ham3388

    Thread Starter Member

    Jul 3, 2012
    97
    3
    If this is the case then
    Vceo should be more than
    50% × 10.64 =16 v

    Both BC 109 and BC 107 satisfy our requirement .
    Which one should I select in this case?
    Since there is no price limitations I think I should select BC 107 since its rating is greater.
    What you suggest?
     
  19. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Use the collector and emitter resistance values to calculate a max current if transistor shorted.
    About 37 mA. This further limits your transistor choices to just one with parameters exceeding in-circuit conditions.
     
  20. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    But 10.64V is not the maximum Vce that the transistor would see in the circuit. Vce_max appear when BJT is on the edge of cur off. In real life we pick BJT with Vce0 > Vcc.
     
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