Question

A transistor is used in common - emitter mode on an amplifier circuit . When a signal of 20 mV is added to the base - emitter voltage , the base current changes by 20 mu A and the collector current changes by 2 m . The load resistance is 5 k Omega calculate (a) the factor beta (b) the input resistance R_"in" (c ) the transconuctctance and (d ) the voltage agin .

Answer 1

(a) The factor beta is :

Given : ∆Ic = 2mA , ∆Ib = 20μA, RL = 5kΩ and ∆Vbe = 20 mV

Beta factor is given as,

β = ∆Ic / ∆Ib = 2 mA / 20μA = 100

(b) The input resistance,

Rin = ∆Vbe / ∆Ib = 20 mV / 20μA

= 1 kΩ

(c) The transconuctctance = 1 / Rin

= ∆Ib / ∆Vbe = 20μA / 20 mV

= 0.1 mho

(d ) the voltage gain,

= ∆Vce / ∆Vbe

∆Vce = RL×∆Ic = 5 kΩ × 2 mA = 10V

Voltage gain = 10V / 20 mV = 500