hfe versus Beta

Discussion in 'Homework Help' started by Konstabel, Jan 31, 2008.

  1. Konstabel

    Thread Starter Active Member

    Jan 31, 2008
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    Can anyone please explain to me the difference between the beta value (Ic/Ib) and the hfe value found on a datasheet. I am trying to design a pnp switch and don't know which value to use in my calculations.

    Thanks
     
  2. mentaaal

    Senior Member

    Oct 17, 2005
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    I think that beta is the current gain for a particular circuit. This depends on things like hfe and resistance values. Hfe is the forward current gain of the particular transistor and can vary from transistor to transistor.
     
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Firstly hfe is one of a suite of h parameters and refers only to the common emitter configuration. You can call it the AC current gain.

    All the h parameters are AC parameters.
    Lower case is used to denote small signal operation. (hfe)
    Upper case is used to denote large signal conditions. (hFE)
    Small signal means signal is not large enough to take the transistor where the characteristics are significantly changed.
    Large signal obviously does and is what you need for switching as the transistor will go from full off to full on.

    hFE is always less than hfe.

    Beta is a static (DC) parameter the DC current gain,which gives a rough idea of the ac gain, but is much easier to measure. It is what is indicated on simple transistor tester.
     
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  4. Konstabel

    Thread Starter Active Member

    Jan 31, 2008
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    Thanks for the answers.

    Just to make sure. Then when using a transistor as a on/of switch I will be using the beta value in my calculations?
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Don't know what you are calculating?
    When you use a transistor as a switch the gain (so long as it has some) doesn't usually enter into it.

    Beta and hfe are for analogue circuit calculations. When you use a transistor as a switch your first approcimation is

    Vout = zero (transistor off)
    or
    Vout = Vcc (transistor on)

    Important parameters are switching time (on and off) and Vcesat the saturated voltage between the collector and emitter, which determines the minimum low.

    In particular if operating the transistor in the saturation region (switching) the relationship

    Ic = beta times Ib

    Is not true.

    Use the equation

    Icsat = Vcc/(Rc + Re)
     
  6. Konstabel

    Thread Starter Active Member

    Jan 31, 2008
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    The calculations I am referring to is in designing the switch circuit when the resistor values are unknown but the base current is known.

    To use the formula you gave (Icsat = Vcc/(Re+Rb), why does it not take into calculation the Veb_sat value?

    Just to make sure, where is Vcc measured?
     
  7. studiot

    AAC Fanatic!

    Nov 9, 2007
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    I expect this is the analysis you are looking for.
     
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