First slide

Transistors

Question

A transistor is used in common – emitter mode in an amplifier circuit. When a signal of 20 mV is added to the base – emitter voltage, the above current changes by $20 μ A$ and the collector current changes by 2 mA. The load resistance is $5 k Ω$ . Calculate
a) The input resistance $R_{B E}$ ,
b) The transconductance and
c) The voltage gain.

Moderate

A

$100 Ω, 10 m h o, 10$
B

$100 Ω, 1 m h o, 50$
C

$1000 Ω, 0.1 m h o, 500$
D

$50 Ω, 10 m h o, 100$

Solution

a) The input resistance $R_{B E} = \frac{Δ V_{B E}}{Δ I_{B}} = \frac{20 m V}{20 μ A} = 1 k Ω$
b) Transconductance    $= \frac{Δ I_{C}}{Δ V_{B E}} = \frac{2 m A}{20 m V} = 0.1 m h o$
c) The Change in output voltage is $= R_{L} Δ I_{C} = (5 k Ω) (2 m A) = 10 V$
The applied signal , voltage = 20mV
Thus , the voltage gain is, $\frac{V_{o u t}}{V_{i n}} = \frac{10 V}{20 m V} = 500$

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