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DC current gain (hFE)

  1. Jan 18, 2016 #1
    OK. On datasheet of transistor 2N4401 there are many DC current gains (hFE). Which one is useful. I want to use it over 5V and 60mA.
     

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  3. Jan 18, 2016 #2

    BvU

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    Table only shows minimum hFE and one maximum. there may be some non-linearity at low current but you can expect at least around 100. If you need the precise value you'll have to measure it on your actual transistor.
     
  4. Jan 18, 2016 #3

    phyzguy

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    You need to be more specific. 5V and 60mA where? What are the bias conditions, specifically Vbe, Vce, and Ib? Even after you have these, you need to realize that current gain varies a lot from transistor to transistor. Bipolar transistor amplifier circuits are usually designed so that the gain is determined by the passive components (usually resistors) and not the transistor itself.
     
  5. Jan 18, 2016 #4

    LvW

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    Just for your understanding: The parameter hfe is NOT identical to the DC current gain "B".
    The parameter hfe is a small-signal parameter resulting from the small-signal 4-pole model of the BJT.
     
  6. Jan 19, 2016 #5

    meBigGuy

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    This sheet has two different gain specifications

    hFE (DC current gain) which is beta and has many specified conditions.
    hfe (Small-Signal Current gain) at one specified condition, which the the small signal gain.

    You can probably find a better 4401 data sheet that has some graphs you can interpolate.

    In order to help we need a little more information about your circuit.

    If you don't like the gain values, look at the 2N2222
     
  7. Jan 19, 2016 #6
    Ok. Forget about 2N4401. We can take this simple circuit

    Lamp is 30mW 5V
    Battery 5V
    Beta=100
    Which transistor can i use in this circuit and how to find it.
    On datasheet there is many hFEs.
     

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  8. Jan 19, 2016 #7

    LvW

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    Yes - agreed. Here in germany, we do not discriminate between hFE and hfe.
    We are using the small-signal values hfe (or h21) and the DC current gain B.
     
  9. Jan 19, 2016 #8

    meBigGuy

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    In a circuit like that you want Vce to be as low as possible. That means you should run in saturation. Notice that in the high hFE specs there is significant Vce.

    In the spec for Vce(sat), notice two things. Vce is very low, and Beta is assumed to be 10. Whenever you want to use a transistor as a switch, you assume a Beta of 10. That is, you overdrive the base to get Vce as low as possible. That reduces dissipation in the transistor, and maximizes the voltage across the load.

    All transistor specifications quote Vcs(sat) at a forced beta of 10.

    If you need low base current, you should change to a PMOS power fet. It will have 0 gate current (except when switching) and lower Vds(on).
     
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