Common emitter amplifier calculation

Discussion in 'General Electronics Chat' started by mahela007, Sep 30, 2011.

  1. mahela007

    Thread Starter Active Member

    Jul 25, 2008
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    After figuring out how to properly connect a transistor, I want to make the necessary calculations to use it as an amplifier.
    The datasheet for the transistor shows a very wide rance.. from 130 to 520
    http://www.alldatasheet.com/datasheet-pdf/pdf/108447/ETC/2SC828.html

    How do I determine which beta value to use, so that I can calculate at what point the transistor will become saturated?
    (I plan to calculate the maximum base current required for saturation by first calculating the maximum current that can flow from Collector to Emitter and then dividing that by the beta value)
     
  2. Hi-Z

    Member

    Jul 31, 2011
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    I think you'll need to give more information regarding how you wish to use the transistor, and a circuit diagram would help. Why are you interested in saturated operation?

    It's perfectly normal for a transistor to have a wide variation in current gain, and in practice it would be very unusual for this to present problems.
     
  3. Hi-Z

    Member

    Jul 31, 2011
    157
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    It occurs to me that you may be talking about a saturated switch, rather than amplifier. If so, yes, use that calculation, but do make sure you have plenty of base current to ensure a nicely saturated transistor. Usually, you'll have more problems if your saturated switch isn't properly saturated than if the base current is overdone (though you might have to watch out for slow switch-off if you're very heavily saturated).
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    The Oriental transistor has a wide range of current gain. But an American 2N3904 transistor and many more or a European transistor with an A, B or C suffix have a much narrower range like 100 to 300.

    A resistor in series with the emitter and a voltage divider for the base voltage reduce the effects of the range of current gain.
     
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  5. mahela007

    Thread Starter Active Member

    Jul 25, 2008
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    I want to know the base current level for which my amplifier circuit will saturate. My problem is that I don't know what beta value to use in my calculation because the datasheet gives such a large range of beta values.. Could you please show me how you would calculate it?
     
    Last edited: Oct 1, 2011
  6. Audioguru

    New Member

    Dec 20, 2007
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    BETA is used when the transistor is a linear amplifier with plenty of collector to emitter voltage so it is NOT saturated.

    The datasheet for most transistors show its saturated collector to emitter voltage when its base current is 1/10th its collector current.
     
  7. mahela007

    Thread Starter Active Member

    Jul 25, 2008
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    Yes.. but to find out the base current that would saturate the transistor, the beta value would have to be known right?
     
  8. debjit625

    Well-Known Member

    Apr 17, 2010
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    Audioguru already said about the 1/10 stuff i.e.. to saturate the transistor you need 1/10 of collector current for base.Actually that gives you the beta value at saturation i.e.. 10 times.For example your transistor's max collector current is 100mA then ,if you bias the transistor using 10mA base current i.e 1/10 th of collector current then your transistor will be saturated.

    Many European transistors will have this value of 1/20 of collector current like BC547.

    Good Luck
     
  9. upand_at_them

    Active Member

    May 15, 2010
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  10. Audioguru

    New Member

    Dec 20, 2007
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    But that is WRONG because the author does not read datasheets.

    The datasheets for nearly all American small transistors say the base current should be 1/10th the collector current to saturate the transistor and the datasheets for nearly all high gain European small transistors say the base current should be 1/20th the collector current even if the Beta is as high as 900.
     
  11. mahela007

    Thread Starter Active Member

    Jul 25, 2008
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    Ok.. now I get it. Many thanks!
     
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