transistor as a switch

moslty transistor are used in following application

transistor as a switch

transistor work as a switch
if we see the picture we have three region there
cutoff
active
saturation

cutoff
in cut off region
The input and Base are grounded (0v)
Base-Emitter voltage VBE < 0.7V
Base-Emitter junction is reverse biased
Base-Collector junction is reverse biased

Transistor is "fully-OFF" (Cutoff region)
No Collector current flows ( IC = 0 )
VOUT = VCE = VCC = "1"
Transistor operates as an "open switch"
Saturation
•The input and Base are connected to VCC
•Base-Emitter voltage VBE > 0.7V
•Base-Emitter junction is forward biased
•Base-Collector junction is forward biased
•Transistor is "fully-ON" (saturation region)
•Max Collector current flows (IC = Vcc/RL)
•VCE = 0 (ideal saturation)
•VOUT = VCE = "0"
•Transistor operates as a "closed switch"