First slide

Transistor

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 rnho and current gain 20, the voltage gain will be

Very difficult

A

$\frac{1}{3} G$
B

$\frac{5}{4} G$
C

$\frac{2}{3} G$
D

$1.5 G$

Solution

Voltage gain= Current gain $\times$ Resistance gain
$A_{υ} = β \times \frac{R_{o u t}}{R_{i n}} T r a n s c o n d u c \tan c e, g_{m} = \frac{β}{R_{i n}} o r R_{i n} = \frac{β}{g_{m}} ∴ A_{υ} = g_{m} R_{o u t} F o r f i r s t c a s e, A_{υ} = G, g_{m} = 0.03 m h o, β = 25 ∴ G = 0.03 R_{o u t} - - - (i) F o r \sec o n d c a s e, A_{υ} = G^{'}, g_{m} = 0.02 m h o, β = 20 ∴ G^{'} = 0.02 R_{o u t} - - - (i i) D i v i d e (i i) b y (i), w e g e t \frac{G^{'}}{G} = \frac{2}{3} o r G^{'} = \frac{2}{3} G$

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