Characteristics of common emitter [CE] configuration
(1) Active Region: In this region collector junction is reverse biased and emitter junction is forward biased. It is the area to the right of VCE = 0.5 V and above IB= 0. In this region transistor current responds most sensitively to IB. If transistor is to be used as an amplifier, it must operate in this region.
(2) Cut Off: In this region collector junction is reverse biased and emitter junction is reverse biased Cut off in a transistor is given by IB = 0, IC= ICO.
(3) Saturation Region: In this region both junctions are forward biased. Since the voltage VBE and VBC across a forward is approximately 0.7 V therefore, VCE = VCB + VBE = - VBC + VBE is also few tenths of volts. In this region the transistor collector current is approximately given by VCC / RC and independent of base current.