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Finding the Early voltage from the 2N5551 datasheet

  1. Feb 5, 2012 #1
    Is it possible to find the Early voltage (VA) of the 2N5551 based on the information in the datasheet? As it happens, I see that the SPICE model in the datasheet lists this parameter at 100V, but that bypasses the purpose of the question. Thanks.
  2. jcsd
  3. Feb 5, 2012 #2
    You need to find the datasheet that show hoe.
    Knowing hoe we can without difficulty determine Early voltage.
    See this datasheet as a example

    Va = (Ic/hoe) - Vce = (1mA/5uS) - 10V = 200V - 10V = 190V

    But as you can see we have a large manufacture spread. So in practice, if we don't know the exact value we assume a type value Va = 100V
  4. Feb 5, 2012 #3
    Yes, but unfortunately I can't find this hoe in the datasheet.
  5. Feb 5, 2012 #4
    This is normal.
    Why do you need to know Va voltage?
  6. Feb 5, 2012 #5
    Hmm... in the graphic of forward gain x collector current... the increase in forward gain in small signals is due to Early effect... should be able to get it from there, shouldn't it?

    hf = hfo ( 1 + Vce/Va)

    if one matches hf vs Ic, and Ic vs Vce... that should give Va...?
  7. Feb 5, 2012 #6
    In my book, the Early effect is modelled as a resistor, ro, between the collector and emitter:

    ro = (VA +VCE)/IC

    I'm trying to find this value for a simple CE stage based on the 2N5551.
  8. Feb 5, 2012 #7
    Not with this data sheet. Look for another brand that has the Ic vs Vce curve. If you cut the graph and stick on a piece of paper with a lot of room on the left side, then extend the straight portion towards the left, they all should meet at Ic=0 at one point and that's the early voltage Va.

    If it is a common transistor, there should be plenty of manufacturers. I notice different manufacturer give different information.

    And other way if you have good meter, measure it yourself. Set up a bias condition with grounded emitter, then vary the collector voltage and read the current. This is hard as the change are very small. But it's doable.
  9. Feb 5, 2012 #8
    But do you know that
    hoe = 1/ro
    Also when load resistance (Rc||RL) of a CE stage is not such a larger we can ignore ro effect on the voltage gain.
  10. Feb 10, 2012 #9
    Problem is data sheet of normal small signal BJT don't give you the h parameters, only β. Only the RF transistors that give you s-parameters, then you can do transformation.

    Also the output impedance of a transistor is not ro alone as hoe is measure with ib=0. The collector curves are drawn with ib not equal to zero. It depend on the collector current.
    Last edited: Feb 10, 2012
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