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Transistor Forward DC gain

Discussion in 'General Electronics Discussion' started by tahir4awan, Nov 29, 2010.

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  1. tahir4awan

    tahir4awan

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    Nov 29, 2010
    First Hi to All the People because it is first time I am writing to any Forum.
    I am doing B.Tech in Electronics I have read hundred of Books on Transistor but didnt find much details regarding basic concept.
    For example Transistor 2N3904 has a maximum Ic 200mA and generally in all the books its DC gain is given as 100 but when you read data sheet the gain table is very complex. Ic from 0.1mA to 50 mA the gain is 30 to 40 with 100 at Ic 10mA.
    My point is we all know there is no calculation possible without DC gain even in voltage divider bias if you want to calculate base current you have to use DC gain but when I see variations in the DC gain how can we use DC gain in calculations. If the gain is maximum at 10 mA then why the manufacturer made current rating to 200mA.
    Second why there is no DC gain stated above 50mA in data sheets how come we know what would be the gain above 50mA.
    Please do help me....
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    All GOOD biasing arrangements have some form of negative feedback so that variations in hfe, Vbe, etc. are cancelled out to a significant degree.

    You mention a voltage divider used to bias a transistor. In these cases you will almost certainly see an emitter resistor. This resistor provides negative feedback by increasing the voltage on the emitter (thus reducing Vbe) as the emitter current increases. The overall effect is that the bias remains relatively stable over significant variations in temperature and the properties of the transistor itself.

    The variation of gain with Ic is just a reflection of the fact that transistors are not perfectly linear.

    The datasheet I found (here) gives hfe for iC up to 100mA. Note that the figures you quoted were the minimum. I would estimate that the minimum for Ic of 200mA to be 10 to 15, but would you really want to use the transistor so close to its maximum rating?

    For a 200mA load I would be looking at a transistor that has better gain at higher Ic. The 2N2222 is an example. It has a minimum gain of 100 and Ic of 150mA. Note that the gain follows what should be a familiar pattern, falling to a minimum of 30 or 40 at 500mA. Note that the drawback for higher gain at higher Ic is that you may have lower gain at lower Ic.

    A similar effect is noticed with rated power dissipation. As the rated dissipation goes up, the gain tends to go down.

    And also with frequency, Note that these datasheets give a gain-bandwidth product (fT). An fT of 300MHz at f=100MHz implies that the gain falls to 3 at this frequency.

    After reading 100 books on transistors, I am quite surprised you haven't come across this. Although, in your favour, most texts tend to deal with such stuff in such a way that the mathematics can obscure the meaning unless you are completely on top of the math.
     
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