In a common-emitter amplifier, the purpose of the emitter resistor is to
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Answer: C
The emitter resistor is there to provide thermal stability.
If temperature rises, the collector current tends to rise with it, because leakage current and beta increase. The extra current raises the voltage across the emitter resistor, which lowers the base-emitter voltage and pulls the current back. This negative feedback holds the Q-point steady and prevents thermal runaway.
An unbypassed emitter resistor actually reduces gain and raises the input impedance, so 'increase gain' and 'reduce input impedance' are the reverse of what happens. It does not significantly reduce the output impedance.
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