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How to get hFE value?

Discussion in 'Electronic Basics' started by NoClue, Dec 6, 2006.

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  1. NoClue

    NoClue Guest

    Hi,

    I made a sample circuit and I want to calculate the value of hFE. How
    can I get the value.
    I know hFE=Ic/Ib and Ib is (Vcc-Vbe)/Rb which is about 100uA. In order
    to get Ic, I have to know hFE and a sample circuit is not the same
    condition from datasheet. If I draw this sample circuit to PSPICE, it
    give me the Ic so that I can calculte the value of hFE. But how, what,
    and who know to get hFE without using PSPICE.


    +------+-----+
    | | |
    | | |
    Rb Rc |
    | | |
    | /C +
    +----|B V
    \E -
    | |
    | |
    +-----+

    Rb is 343k, Rc is 810, Vcc is 35V, and transistor is P2N2222A.

    thanks for reading.

    clueless Ken.
     
  2. Use 200 for that transistor.
     
  3. John Fields

    John Fields Guest

    ---


    Measure the voltage on the collector, subtract that voltage from the
    supply voltage and divide that by the collector resistor's
    resistance. That will be the collector current. Call it Ic.

    Measure the voltage on the base, subtract that voltage from the
    supply voltage and divide that by the base resistor's resistance.
    That will be the base current. Call it Ib.

    Divide Ic by Ib. The quotient will be beta.
     
  4. Dave

    Dave Guest

    Extrapolate using the data sheet. Which says, btw, that at I(c)=150mA,
    V(ce)=10V, min hfe=100, max hfe=300. Hence the previous poster's suggestion
    to use 200.

    hfe varies with current, voltage, temperature, etc. Generally highest
    around the mid-point of voltage and current ranges, lower towards the ends.
     
  5. Phil Allison

    Phil Allison Guest

    "NoClue"


    ** Honest at least ....

    ** You cannot.

    Hfe is a variable parameter of *actual devices* - it varies widely from
    sample to sample and is also significantly affected by the voltage current
    and temperature levels experienced by that device.

    What you have drawn is suitable for TESTING the Hfe figure of individual
    PN2222 devices - since the base current is fixed and the collector current
    can be noted from the voltage across Rc.




    ........ Phil
     
  6. chris

    chris Guest

    The simplest and most accurate way to determine the hFE for the
    transistor you are using is to simply measure the current flowing
    through Rc and Rb. Then divide Rc/Rb.
    You could as suggested else-where measure the voltage across the
    resistors, however you then open yourself to errors when you remember
    the tolerance values applied to the resistors.

    Chris
     
  7. Phil Allison

    Phil Allison Guest

    ** Top Posting Groper Alert !!



    ** Better try Ic / Ib.


    ** With standard 1% resistors and a decent DMM, the result will be within
    1% which is likely to be as good or better than using the same DMM as a
    current meter.

    When matching the Hfe of two or more devices, accurate test figures are
    irrelevant.



    ....... Phil
     
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