For the given transistor network in figure, find the working mode of transistor -

Concept:

Active mode:

In the active mode, one junction (emitter to base) is forward biased and another junction (collector to base) is reverse biased.

Method to check region of operation:

Step 1 : Assume BJT to be in saturation region (V_CE = 0.2 V), and calculate I_Csat from the circuit.

Step 2: Calculate I_Bmin = ICsat/ β 

Step 3: Calculate I_B from the given circuit.

Step 4: If,  IB > IBmin → Saturation region

                 IB < IBmin → Active region

Analysis:

Assume,

V_BE = 0.7 V

V_CEsat = 0.2 V

ICsat = ( V_CC - V_CEsat)/ R_C

⇒ ICsat = ( 9 - 0.2)/2 K

⇒ ICsat = 4.4 mA

IBmin = ICsat/ β = 4.4/50 = 0.088 mA

I_B = ( 9 - 0.7)/300K = 0.0276 mA

 IB < IBmin → Active region

Additional Information Saturation mode:

In the saturation mode, both the junctions of the transistor (emitter to base and collector to base) are forward biased.

Cutt-off mode:

In the cutoff mode, both the junctions of the transistor (emitter to base and collector to base) are reverse biased.

Inverted mode:

In the inverted mode ( Reverse active mode), one junction (emitter to base) is reverse biased and another junction (collector to base) is forward biased.