Question

The change in emitter current by 2.1mA results in change of 2mA in the collector current and a change of 0.05V in the emitter base voltage. The input resistance is

• A. 5K$\Omega$
• B. 3K$\Omega$
• C. 1K$\Omega$
• D. 0.5K$\Omega$

Answer 1

The correct option is D 0.5K$\Omega$

Given that:
$\Delta I_e=2.1mA=2.1\times {10}^{-3}A$
$\Delta I_c=2mA=2\times {10}^{-3}A$
For transistors,
$\Delta I_e=\Delta I_b+\Delta I_c$
$\Delta I_b=\Delta I_e-\Delta I_c=2.1\times {10}^{-3}-2\times {10}^{-3}=0.1\times {10}^{-3}A$
Hence, input resistance is
$R_i=\frac{\Delta V_{eb}}{\Delta I_b}$
$R_i=\frac{0.05}{0.1\times {10}^{-3}}=0.5K\Omega$