An NPN transistor is used in common emitter configuration as an… | JEE Main 2019 (9 Apr, Shift 1)

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Junction Transistor

An NPN transistor is used in common emitter configuration as an amplifier with 1 k $\Omega$ load resistance. Signal voltage of 10 mV is applied across the base-emitter. This produces a 3 mA change in the collector current and 15μA change in the base current of the amplifier. The input resistance and voltage gain are:

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Answer: (c)

Input resistance: $r_i = \dfrac{\Delta V_{BE}}{\Delta I_B} = \dfrac{10\ \text{mV}}{15\ \mu\text{A}} = 0.67\ \text{k}\Omega$
Output voltage change: $\Delta V_o = \Delta I_C R_L = 3\ \text{mA} \times 1\ \text{k}\Omega = 3\ \text{V}$
Voltage gain: $A_v = \dfrac{3\ \text{V}}{10\ \text{mV}} = 300$
So the answer is (c) $0.67\ \text{k}\Omega$ , 300.

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