BJT Regions of Operation

Cut-off, active, saturation and reverse-active — which junctions are biased which way.

Intermediate

Region	B–E junction	B–C junction	Behaviour
Cut-off	reverse / off	reverse	switch off, $I_C \approx 0$
Forward active	forward	reverse	amplifier, $I_C = \beta I_B$
Saturation	forward	forward	switch on, $V_{CE} \approx 0.1$ –0.3 V
Reverse active	reverse	forward	rarely used

Amplifiers bias the transistor in the active region; digital switches flip between cut-off and saturation.

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