... Your meter will give the hFE at a specific collector current ...
Yes, it should be that way. However, what is the value of that specific current ? i guess the manual of my DMM should specify it, right?? well, guess what ... it didn't :lol:
If hFE varies with Ic, then what people usually say "
to calculate Ic, multiply hFE by Ib" is wrong, isn't it ? If that is wrong, and i guess so, then what is the mathematical equation for Ic ?? Actually i don't have a specific circuit that i want to calculate the current for .. i just want to know how to do so in general.
Also, i've tried to make the Ic - Ib plot. What i did is that i've started gradually from a zero Ib upto about 10 uA, measuring Ic for each Ib. My design sets Ic(sat) to 10 mA (to prevent the transistor from oversaturation, as there is no hFE of 1000
) .. my first Ib was 1uA, the corresponding Ic was 148uA, which means that hFE is 148. Then, i've tried the rule that says "
to get Ic, mutiply Ib by hFE" so i did that for the next value of Ib .. i've set the Ib to 2uA multiplied it by 148 .. and, tada: Ic = 296uA and when i measured it, Ic was exactly 296uA ... A SHOCK (for me at least :lol
!! why hFE didn't change?? is it possible that the hFE for Ic = 140uA is the same as for Ic=296uA ?? Well, experimentally, yes. Theoretically, i don't know! Maybe there is no change in hFE when Ic is in uA's scale ?! Because if you look at it, most datasheets that i've read indicate hFE values for Ic's in the mA range .. could that be a reason ?! I don't know .. help me out everyone :cry:
... You should design circuits to make them independent of hFE variations ...
Interesting!! How could i do that (using transistors only) ? :roll:
Thanks again Nigel..