Saturation in transistors (BJTs) - why and how

[KrisBlueNZ]

KrisBlueNZ submitted a new resource:

Saturation in transistors (BJTs) - why and how - The reason you would saturate a BJT, and calculating base current and resistor values

When a bipolar junction transistor (BJT) is used to switch a load (e.g. a relay, LED, etc) ON and OFF, it is most often operated as a "saturated switch". This article explains saturation in BJTs - why it is used, and how to calculate the base resistor to ensure saturation.

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[hevans1944]

At least one reference defines saturation occurring when the base-emitter junction is forward biased and the base-collector junction also becomes forward biased, as a result of the decrease in collector voltage. I think it does no harm to have a little "too much" base current, just to guarantee saturation does occur.... shooting for Vce in the neighborhood of 0.2 V.

 

[KrisBlueNZ]

Right, that's the thrust of the resource - that you need "too much" base current.

I mentioned forward-biasing of the base-collector junction as one way to define saturation, but no matter how you define it, there is no clear demarcation between "not quite saturated" and "saturated". Even if you use the base-collector forward bias criterion, I don't think it's right to say saturation "occurs" as soon as any current flows that way. Perhaps you could say it occurs when I_BC reaches or exceeds I_BE but that's also just arbitrary.

 

[hevans1944]

Yeah, the transition from linear operation to "saturation" is not abrupt. Brute force is the way to go as long as you don't fry the base-emitter junction.