Question

In a common base configuration, the emitter current changes by 5mA when the emitter current is changed by 200mV at a fixed collector to base voltage. The input resistance is
A). $40\Omega$
B). $1000\Omega$
C). $2.5\Omega$
D). $4\Omega$

Answer 1

Hint: The configuration in which the base of the transistor is common to both collector and emitter terminals is called Common base configuration. Use the formula for input resistance in a common base configuration. Input resistance is the ratio of change in input voltage to change in input current. Substitute the value for input current and input voltage and obtain the input resistance.
Formula used:
$r= \dfrac {\Delta V}{\Delta I}$

Complete step-by-step solution:
Given: V = 200mV = $200 \times {10}^{-3} V$
            I = 5mA = $5 \times {10}^{-3} A$
For transistors in common base configuration, resistance is given by,
$r= \dfrac {\Delta V}{\Delta I}$ …(1)
Where, $\Delta V$ is the change in input voltage
             $\Delta I$ is the change in input current
Substituting value in equation. (1) we get,
$r= \dfrac {200 \times {10}^{-3}}{5 \times {10}^{-3}}$
$\therefore r= 40\Omega$
Thus, the input resistance is $40\Omega$.
Hence, the correct answer is option A i.e. $40\Omega$

Additional Information:
There are three bipolar junction configurations. They are Common-Base (CB) configuration, Common-Emitter (CE) configuration, and Common-Collector (CE) configuration. These configurations vary on the basis of voltage gain, current gain, power gain, input impedance, output impedance, etc.

Note: The ratio of collector current to the emitter current gives current gain. Current gain is less than or equal to unity for Common-base configuration. This configuration has high output impedance and low input impedance. Resistance gain i.e. ratio of output resistance to low resistance is high for the common base configuration.