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What is the output resistance, When we observe in a common emitter circuit the collector current increase from $9.5 \;mA\;to 10\;mA$ When the collector voltage increase from $2.5 \;to \;6v$ When base current is kept constant .

$(a)\;7.5 K\; \Omega \\ (b)\;8.5 k \; \Omega \\ (c)\;3.2 k \;\Omega \\ (d)\;7.0 k\; \Omega$

Output resistance $r_0=\large\frac{\Delta V_{cE}} {\Delta I _c}$
$\qquad= \large\frac{(6- 2.5)v}{(10-9.5 )m A}$
$\qquad= \large\frac{3.5 \;v}{0.5 mA}$
$\qquad= 7 k \Omega$
Hence d is the correct answer.