Discussion
A transistor operates at \( I_{C} = 1 \ \mathrm{mA} \). Its transconductance is about
Answer: D
Dividing the collector current by the thermal voltage:
\[ \begin{aligned} g_{m} &= \frac{1 \ \mathrm{mA}}{26 \ \mathrm{mV}} \\ &\approx 40 \ \mathrm{mS} \end{aligned} \]
giving about 40 millisiemens, or equivalently a resistance of 26 ohms seen looking into the emitter. Twenty-six millisiemens confuses the thermal voltage's numerical value with the answer. The rule of forty millisiemens per milliamp is worth memorising, since it makes most small-signal gain estimates a matter of mental arithmetic.
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