Question

In a common emitter (CE) amplifier having a voltage gain G, the transistor used has transconductance 0.03 mho and current gain 25. If the above transistor is replaced with another one with transconductance 0.02 mho and current gain 20, the voltage gain will be
(a) 1.5 G
(b) $\frac{1}{3}G$
(c) $\frac{5}{4}G$
(d) $\frac{2}{3}G$

Answer 1

Amplifier -  A transistor is used as an amplifier , wherein  in such circuit output is amplified as compared to input,

Thus,

Voltage gain ∆ᵥ = β Rₒᵤₜ / Rᵢₙ

G = 25 Rₒᵤₜ / Rᵢₙ ….(1 eq)

Transconductance gₘ = β / Rᵢₙ

Rᵢₙ = β / gₘ = 25 / 0.03

Putting the value of Rᵢₙ in eqn.(1)

G = 25 Rₒᵤₜ / 25 x 0.03 …(2 eq)

G’ = 20 Rₒᵤₜ / 20 x 0.02 …( 3 eq)

From eqs. (2) and (3)

Voltage gain of new transistor G’ = 2/3 G