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How to interpret the hFE(beta) in a transistor datasheet?

Discussion in 'Electronic Basics' started by nabi, Feb 22, 2007.

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  1. nabi

    nabi Guest

    I'm analyzing a ciruit which contains TIP31 NPN transistor.
    The hFE (current gain) on the datasheet is 25 (Vce = 4V, Ic = 1A),
    10 to 50 (Vce = 4V, Ic = 3A).
    The circuit is about a battery charger and it handles various voltage,
    i.e.
    The hFE on the datasheet changes in different voltages or it maintains
    same range?

    I looked at the graphe about hFE but it only shows of 4V test with Ic
    variation.

    My question is how do I estimate the hFE of a transistor which is used
    different setting compared to the datasheet?
     
  2. chuck

    chuck Guest

    Nabi, the most direct route is to simply measure hFE at the operating
    point you are contemplating.

    Small signal current gain varies considerably from transistor to
    transistor. Circuit designers often employ negative feedback to limit a
    transistor's hFE so that mass-produced circuits will function
    more-or-less the same despite variability in transistor characteristics.

    Chuck
     
  3. Phil Allison

    Phil Allison Guest

    ** Groper alert:


    ** Find the published curve of Hfe versus Ic.

    My data book shows it is over 100 up to 0.5 amp Ic.

    Then falls to about 25 at 3 amp Ic.

    This is at a fixed Vce of 2 volts.

    More Vce will only improve the figure.

    Vce sat is about 1.2 volts at 3 amp Ic with 300 mA Ib.

    Simple.



    ......... Phil
     
  4. Rich Grise

    Rich Grise Guest

    The more "sophisticated" way would be to design the circuit such that hFE
    (aka bets) variations don't really make all that much difference. Like,
    design for the low value, but arrange the circuit (feedback, etc) so that
    using a higher-gain transistor will still give the same answer.

    In other words, design it out. :)

    Good Luck!
    Rich
     
  5. Marra

    Marra Guest

    The HFE changes with voltage and current.
    I would always allow an overhead of 50% on any design I made to be
    safe.

    www.ckp-railways.talktalk.net/pcbcad21.htm
     
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