Question

With the help of necessary circuit diagram, describe briefly how n-p-n transistor in CE configuration amplifies a small sinusoidal input voltage. Write the expression for the ac current gain

Answer 1

n-p-n Transistor as an amplifier:

The operating point is fixed in the middle of its active region.

An ac i/p signal v_i is superimposed on bias V_BB (dc). The o/p is taken between the collector and the ground.

Applying Kirchhoff’s law to the output loop:

V_CC = V_CE + I_CRC    

​If Vc is the collector voltage then,

Vc = V_CE − I_CR_C    ...(A)

When the input signal voltage is fed to the emitter base circuit, it will change the emitter voltage and hence to the emitter current, which in turn will change the collector current. Due to this the collector voltage V_C will vary in accordance with relation (A). This variation in collector voltage appears as amplified output. 

V_BB = V_BE + I_BR_B

vi ≠ O

Then, V_BB + v_i = V_BE + I_BR_B + ΔI_B (R_B + r_i)

`r_i=((DeltaV_(BE))/(DeltaI_B))_(V_`

`:.v_i=DeltaI_B(R_B+r_i)`

`=rDeltaI_B`

`beta_`