True, the actual beta is fixed for a given transistor. Forced beta means you use a beta lower than the minimum beta (and certainly lower than the maximum you quote) and thus a higher base current to insure that the transistor is well turned on for minimum voltage drop across the collector emitter terminals. Normal beta values are used when operating the transistor as a linear amp, not a switch.
I would always use a beta value no greater than half the minimum value data sheet value when using the transistor as a switch to have a low switch voltage drop when on.
Okay so I am trying to understand this...
Here is the datasheet I am looking at:
http://www.datasheetcatalog.org/datasheet/SGSThomsonMicroelectronics/mXyzzyw.pdf
Now, my understanding is that as you increase the base current, you are going from cut-off to active to saturation. So now looking at the datasheet, you can see that as Ic increases the beta increases. So does this mean that beta depends on the amount of current required to drive the load? So for example, if my load requires a current of 2.2mA (Ic) then the beta is approximately 50 (according to the datasheet)? And so to reach the 800mA max Ic I would need 16mA at the base (16 x 50 = 800)? No, this does not make sense because once I start increasing Ic, the beta changes. See this is what doesn't make sense to me.
crutschow, you are saying that you use a higher base current to have a lower than minimum beta... but the datasheet says that as you increase current the beta increases as well....
Oh wait, I think I know what you guys mean now. By having high base current, you are putting the transistor in saturation and in saturation your gain is flat. So you can keep increasing the base current without increasing Ic and thus your beta, or ratio between base and collector current, is much smaller. Is that it? If so, then how do you know, mathematically, when the transistor is in saturation? How can I calculate this?
A lot of websites have you calculate Ib = Ic_required/beta to find the base current needed to put the transistor in saturation but this does not make sense since the amount of base current needed to put a transistor in saturation does not depend on the load. Or am I wrong?
Sorry for going off on a rant. Just thought I'd type all my thoughts so that it made more sense as to where my confusion lies. Thank you all for your replies thus far.