Question

In a common base configuration, the emitter current changes by $5mA$ when emitter voltage 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

The correct option is A $40$$\Omega$

Given that:
V = $200mV=200\times {10}^{-3}V$
I = $5mA=5\times {10}^{-3}A$
For transistors in CB mode, the dynamic resistance is the ratio of change in emitter voltage to the change in emitter current.
$r_d=\frac{\Delta V}{\Delta I}$

$r_d=\frac{200\times {10}^{-3}}{5\times {10}^{-3}}=40\Omega$