The rule of thumb to force the transistor into saturation is Ib = Ic / 10.
Its a term called forced beta in the industry. Generally speaking when
we use a transistor as a switch we want the Vce to go as low as possible,
so that its power dissipation is kept low and the switch acts as close as
possible as a short, like relay contacts typically do.
So the base R, Rb =~ ( Voh - Vbe ) / Ib =~ ( Voh - Vbe ) / [ Ic / 10 |
Voh comes from micro sdatasheet, when an outpuit is turned on to high
logic level, and given at a specific current. Some datasheets show a typical
graph of Voh versus load current.
Typical curves that apply (2N3904 as example) :
As you can see in the graph, 10 mA case, we get a low Vce when we force 1 mA into
the base. Any more base current has very little egffect, eg. no more than Ic / 10 is
needed in most designs.
Note some transistors have even lower Vce when collector and emitter are reversed,
but takes more base current then Ic / 10 to get that increased performance.
Regards, Dana.