# Choosing the correct transistor HFE value for calculating base current

In my quest as a noob to better understand transistors, I have been reading up on calculating the base current to fully saturate the transistor. The guide that I read says to use the minimum value HFE value but on datasheets I see several minimum HFE values listed for specific collector current and voltage combinations. Should I select the smallest HFE value out of those listed to be on the safe side?

For example, I'm looking at the datasheet for the BD139 transistor (www.fairchildsemi.com/ds/BD/BD135.pdf?. If I had a collector voltage of 5 volts and a load that consumes 60mA should I use the HFE value of 25 to get 2.4 mA base current(I have also read various things about doubling the base current to be on the safe side)?

Should I select the smallest HFE value out of those listed to be on the safe side?

Yes.

However you can choose the lowest one at the collector current you are using.

Yes, that Ib=2.4mA shall saturate the BD135 @Ice=60mA.

pito: Yes, that Ib=2.4mA shall saturate the BD135 @Ice=60mA.

Not necessarily - it depends on what you count as saturation. The only guaranteed figure is the Vce(sat) figure, which is quoted at a specific Ic and Ib. The hfe figures are quoted when the transistor is not saturated. For example, for the the BD135 it is quoted at Vce = 2v. For many other transistors, hfe is quoted at a higher Vce still.

A good rule of thumb is that you should design for Ib = 1/10 of Ic, if you can afford that much base current. Vce(sat) is typically quoted at Ib = 1/10 Ic. If you can't afford that much base current (e.g. because you are switching 500mA), go for at least 1/20 of Ic.

In this case, you are running the BD135 at 60mA, which is much lower than the Ic (500mA) at which Vce(sat) is quoted. So 2.4mA base current will probably saturate the BD135 quite well (you generally need proportionally more base current at larger collector currents). Nevertheless, I suggest you design for 3 to 6 mA.

Thanks for the help guys.