When the transistor is used as an amplifier
1
Emitter-base junction must be reverse biased, Collector-Emitter junction must be forward biased.
2
Emitter-base junction must be forward biased, Collector-Emitter junction must be forward biased.
3
Emitter-base junction must be reverse biased, Collector-Emitter junction must be reverse biased.
4
Emitter-base junction must be forward biased, Collector-Emitter junction must be reverse biased.