In a CE amplifier, how does emitter degeneration affect input impedance?

• A. The input impedance remains roughly rπ.
• B. The input impedance decreases due to feedback.
• C. The input impedance becomes purely resistive.
• D. The input impedance increases due to feedback.

Answer: (D)

Emitter degeneration raises the input impedance of a common-emitter stage through local negative feedback. When you add an unbypassed emitter resistor, the base sees not just the intrinsic base-emitter resistance rπ but also a voltage drop across Re that grows with emitter current. In small-signal terms, the emitter current is about (β+1) times the base current, so the voltage across Re is (β+1) ib Re. The input voltage is Vin = ib rπ + (β+1) ib Re, giving Zin = Vin/ib = rπ + (β+1) Re. Since Re provides additional resistance seen from the input, the impedance increases as Re grows. This increase comes from the negative feedback: changes in base current produce emitter voltage changes that oppose the input signal, reducing transconductance and raising input impedance. In midband, with purely real components, Zin is largely real, though/parasitics can introduce a small reactive part.