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The current $i_b$ through the base of a silicon $npn$ transistor is $1+0.1\cos(10000\pi t)\:mA$. At $300\:K$, the $r_\pi$ in the small signal model of the transistor is

1. $250\:\Omega$
2. $27.5\:\Omega$
3. $25\:\Omega$
4. $22.5\:\Omega$

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