Suppose I have this BC635 transistor, if I look at hFE then I'd say that the lowest value is 25 from that table in the Electrical Characteristics category, but if I look at the Figure 3. Base-Emitter Saturation Voltage from where I get the beta characteristic, on the right side of the plot beta is 10.
Now I only learned about the beta characteristic in school, not about hFE, but I read online that they are the same thing, although this is not true judging from the datasheet I linked.
The question is:
What should I use in my calculations for the base current if I want to use a transistor as a simple DC switch, beta from the plot or hFE?
What's more troubling is that some datasheets do not have those plots, what should I do there?
Best Answer
If you are using the transistor as a saturated switch then you decide what Ic/Ib should be.
The saturated characteristics of the transistor are guaranteed at Ic/Ib = 10. Most often you'll use a bit less drive, maybe Ic/Ib = 20 unless you are very close to the limits of the transistor.
hFE is gain in an unsaturated condition (as an amplifier) with a relatively huge Vce (2V in this case). It is a poorly controlled parameter and varies a lot from part-to-part and with temperature.
You can say that you should have forced \$\beta\$ << hFE (specified at a high Vce) if you want consistent results.
Plots are only an indication of typical behavior, you need to look at the guaranteed limits for design. The plots may help you interpret the guaranteed results at intermediate operating conditions.
So, suppose you want to switch 100mA. hFE (Vce = 2V) is typically around 100, and does not tail off much as you go higher to 200mA (Figure 2) so we can be fairly sure it doesn't do anything weird, but the 100 is only typical. We can see that hFE is guaranteed to be 40 at room temperature and 150mA, so it should be at least 40 at room temperature and 100mA. It might drop 30% at low temperature, so we're left with a guarantee of 28 at low temperature and for a low gain unit. I think I would use Ic/Ib = 10 in this case, not 20.
Now that does not mean that you can't pull a random BC635 transistor off the shelf and use Ic/Ib = 50 and have it work most of the time, but that is not proper design. Don't be that guy.