hFE and data sheets

Discussion in 'Homework Help' started by Steve1992, Apr 30, 2006.

  1. Steve1992

    Thread Starter Senior Member

    Apr 7, 2006
    On data sheets hFE will be quoted at Ic
    eg hFE or β = 110-900 at 1mA(Ic).

    If im working on a common-emitter bipolar amp:

    Ib = Vcc - Vbe / Rb
    So I know the base current.

    Ic = Vcc - Vbe / ( Rb / β )

    What β should I use?

  2. n9352527

    AAC Fanatic!

    Oct 14, 2005
    You are right in noticing that hFE is a function of Ic, and also to some extent Vce. You could find the _typical_ value of hFE at different Ic and Vce from the hFE graph, usually supplied by the manufacturer in the datasheet. However, not the absolute value, as this is device specific.

    This, however, is of little use to real life design of BJT circuit. You can see that the range quoted for hFE is very wide (100-900) and designing for one value of hFE would not guarantee the circuit to work for other value of hFE. You would've needed to handpick each transistor and match their specific hFE to your circuit and still these might change with temperature and biasing changes.

    What you need to do is to reduce the effect of hFE to operation of your circuit by including a negative feedback scheme much like negative feedback on opamp reduce the effect of gain variation and the gain itself. Have a look at improved CE amplifier where there is a feedback resistor Re included. With this configuration, you would need only to worry about the minimum hFE across your biasing settings instead of the exact hFE, which is pretty easy to do.