How can i find the value of Beta of a transistor???

Discussion in 'General Electronics Chat' started by Ehsan Ullah, Sep 2, 2008.

  1. Ehsan Ullah

    Thread Starter New Member

    Aug 18, 2008
    I have got a transistor which has got the following output characteristics. Obtained from the values of Ib and Ic. As Beta = Ib / Ic. so the values of Beta corresponding to it are also given.
    I have couldn't attach the graph but i am giving these values and the value of beta according to it..

    Ib Ic Beta = Ic / Ib
    1.00E-05 2.00E-04 20
    1.00E-05 4.00E-04 40
    1.00E-05 6.00E-04 60
    1.00E-05 1.99E-03 198.747
    1.00E-05 2.27E-03 227.1
    1.00E-05 2.27E-03 226.6

    2.00E-05 5.00E-04 25
    2.00E-05 1.00E-03 50
    2.00E-05 3.50E-03 175
    2.00E-05 3.96E-03 198
    2.00E-05 4.51E-03 225.5

    3.00E-05 7.00E-04 23.33333333
    3.00E-05 1.40E-03 46.66666667
    3.00E-05 3.50E-03 116.6666667
    3.00E-05 4.90E-03 163.3333333
    3.00E-05 5.87E-03 195.6
    3.00E-05 6.38E-03 212.5666667
    3.00E-05 6.72E-03 224.1466667

    4.00E-05 9.00E-04 22.5
    4.00E-05 1.80E-03 45
    4.00E-05 2.70E-03 67.5
    4.00E-05 6.30E-03 157.5
    4.00E-05 7.78E-03 194.5
    4.00E-05 8.23E-03 205.825
    4.00E-05 8.58E-03 214.5
    4.00E-05 8.91E-03 222.72
    What is the actual value of beta and how did you pick this value..
    Last edited: Sep 2, 2008
  2. Wendy


    Mar 24, 2008
    Your numbers are badly flawed somewhere. You show the same base current but many collector currents, which means you have a wiring error or some other basic problem. The formula you show is correct. The transistor has to be in its linear region. Schematic please? How did you arrive at these values.
  3. mik3

    Senior Member

    Feb 4, 2008
    Actually, the beta is not constant even if you amplify DC current because it changes with temperature and Vce.

    If you amplify AC currents the it also changes with the amount of current.

    You can find an approximation to its value or design an almost independent beta circuit.
  4. hgmjr


    Jan 28, 2005
    What parameter in the transistor circuit did you change while keeping the base current constant in order to produce the varying collector currents?

  5. SgtWookie


    Jul 17, 2007
    For a small signal transistor, determine what Ib it takes to maintain an Ic of 10mA when Vce=1v.

    What you appear to be doing at the moment is holding Ib constant, but varying Vce to obtain a wide range of Ic.

    If you want to plot the hFE curve, you could also determine the Ib necessary to achieve Ic of 0.1, 0.5, 1, 5 10, 50, 100mA for Vce=1v.
  6. Ehsan Ullah

    Thread Starter New Member

    Aug 18, 2008
    Thanks for your reply..
    Well obviously i have been changing the Vcc to get different values of Ic for a constant value of Vbb and then i have change vbb and repeated the experiment again?
    But my question is why Beta is not constant?
    I think Beta of a transistor depends on it manufacturing , am i right?
    if beta is veriable than does it affect the amplification property of transistor?
    How can i find the correct value of Beta for a given transistor and use it in my caluclation? becuase in books while solving numericals they have given a value for beta.
    Last edited: Sep 7, 2008
  7. SgtWookie


    Jul 17, 2007
    Beta is going to change.
    You've forced a change by varying Vcc; this is not normal. Usually, Vcc is constant. For obtaining Beta values, Vcc is maintained at 1v, regardless of Ic.
    I've given you a solid method for developing your Beta plots.
    Beta varies considerably, even with transistors in the same lots with the same part number.
    The datasheets will only give guaranteed minimums, and approximate ranges for the maximums.

    If you follow my "recipie", you will then be able to generate accurate and meaningful curves for your transistors. If you don't, you will wind up with garbage.

    If you want more accuracy in your curves, you could also include the "2" values; ie:
    0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 50, 100, 200mA for Vce=1v.
    Last edited: Sep 7, 2008
  8. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    Because beta is such an unreliable parameter (it also varies with temperature) transistor circuits are usually designed so that it is not a dominant factor in circuit performance. A circuit which is designed for a particular value of beta will probably be unstable and unreliable in practice.

    Precise values of beta are usually only used when transistors are required to operate in groups and need to be matched to each other. A good example is the differential pair where transistors of matching parameters are selected and implemented so that changes in beta are effectively canceled.
  9. iamspook


    Aug 6, 2008
    Blocco gives good advice, as do the other posts.

    Beta (also called Hfe) is not a particularly good design parameter. All you need
    to be sure of is that it is large enough at its minimum.

    The bipolar transistor is one of the few devices which seem to get 'better' as
    is gets hot, in that hfe increases with temperature, and hence so does gain.
    The problem with this effect is to maintain the designed operating point so
    that linearity and stability are maintained as the thing gets hot. It gets hot
    from passing current, which makes it hotter. You see the problem? If this is
    not controlled then it will enter thermal runaway and pop.
  10. Audioguru

    New Member

    Dec 20, 2007
    The "Early Effect" causes the collector current of a transistor that has a fixed base current to rise when the collector voltage is increased.
    Google has all the details.