Fixed Bias Transistor Circuits

Few Points:

1. Two batteries v_BB and V_CC used
2. V_BB gives potential to base terminal through R_B.
3. V_CC  supplies power to the collector through R_C.
4. Base resistance R_B  of the order of hundred kilo ohm
5. Collector resistance R_C of the order of kilo ohm.

Aim: To find dc base current I_B, I_C, V_CE and I_E 
For this purpose, we are going to convert  above circuit in electrical equivalent circuit. For making calculation easy, we divide this circuit in two part, (i) Input Part and (ii) Output Part

(I)  Input Part:

Applying KVL:

We get, V_BB = I_BR_B + V_BE

Or  I_B = ( V_BB – V_BE )/ R_B

For Silicon Transistor, V_BE = 0.7 V and For Ge Transistor, V_BE = 0.3 V

(ii) Output Part:

Applying KVL again,

V_CE = V_CC – I_C R_C

We know that,  I_C = β I_B

Stability Factor (S) = ( 1 + β )/(1 – β d I_B / d I_C)

And  in fixed biased, I_B is independent of I_C.

Hence, S = ( 1+ β )

It means collector current (I_C) increases ( 1 + β ) times as much as I_CO .

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