Discussion
A stage runs at \( I_{C} = 0.5 \ \mathrm{mA} \). Its intrinsic emitter resistance is about
Answer: B
The emitter resistance is the thermal voltage over the current:
\[ \begin{aligned} r_{e} &= \frac{26 \ \mathrm{mV}}{0.5 \ \mathrm{mA}} \\ &= 52 \ \Omega \end{aligned} \]
giving about 52 ohms, twice the value at a milliamp because the current is half. The inverse relation means halving the current doubles the resistance and halves the gain a given collector resistor can produce, which is why the operating current is a gain decision as well as a bias one.
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