There are several ways how to know if a transistor is saturated or not. It is very important that if you design a transistor to function as a switch that it should operate in the saturation and cutoff. Operation at cutoff is simply interrupting the bias of the transistor. However operating at saturation is not that easy. You need to
How to drive a transistor into hard saturation? In using a transistor to operate as a switch you must drive it into saturation. Saturation happens when the collector current cannot further increase despite there is base current increase. The saturation level of every transistor varies. The usual range is from 0.7V to ideally zero. For instance for a BC817-25 transistor,
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