Transistor as a Switch

Amplifier versus switch: two different uses of the same device. Section 9.22's amplifier deliberately biases a transistor to sit in the FLAT, LINEAR active region of its output characteristic (Section 9.20), so that its output responds smoothly and proportionally to a varying input. Using the transistor as a SWITCH does the exact opposite: it is deliberately driven to the two EXTREME ends of the same characteristic -- CUT-OFF and SATURATION -- and kept OUT of the linear region altogether, so that it settles, essentially, into only two possible states.

Cut-off: the OPEN-switch state. When the base-emitter junction is not forward biased (e.g. the base voltage is held at or below $0$, or the same as the emitter), no significant base current flows ($I_B \approx 0$), and, since $I_C = \beta I_B$, essentially no collector current flows either ($I_C \approx 0$). With almost no current flowing through the collector load resistor $R_C$, almost the ENTIRE supply voltage appears across the transistor itself, so $V_{CE} \approx V_{CC}$. This state -- no current flowing through the transistor, full supply voltage dropped across it -- is exactly analogous to an OPEN (OFF) mechanical switch.

Saturation: the CLOSED-switch state. When the base is driven with a sufficiently LARGE base current (well beyond what would merely keep the transistor in its linear active region), the collector current rises to the maximum value the external circuit itself can support -- set simply by $I_{C(\text{sat})} \approx V_{CC}/R_C$, essentially INDEPENDENT of $I_B$ from this point onward, since the collector-base junction is now ALSO forward biased and the transistor can no longer respond to further increases in base drive. With this maximum current flowing through $R_C$, almost the entire supply voltage is dropped across $R_C$ itself, leaving only a small residual voltage across the transistor ($V_{CE}$ typically a few tenths of a volt, close to zero). This state -- large current flowing freely, almost no voltage dropped across the transistor itself -- is exactly analogous to a CLOSED (ON) mechanical switch. …