Define input and output impedances of a CE amplifier with and without emitter degeneration.

• A. Without degeneration, input is roughly rπ; with degeneration, input impedance increases due to feedback; output impedance is ≈ RC || ro, modified by transistor parameters.
• B. Without degeneration, input is RC; with degeneration, input decreases.
• C. Both input and output impedances decrease with degeneration.
• D. Input is infinite for CE amplifier.

Answer: (A)

The key idea is that emitter degeneration adds negative feedback through the emitter resistor, which changes how the transistor’s input and output impedances behave.

For the input, without emitter degeneration the AC emitter path is effectively grounded, so the input impedance looking into the base is dominated by rπ. When an emitter resistor is present and not bypassed, the variation in emitter current produces feedback that reflects an additional impedance into the base equal to roughly (β+1)Re. This makes the input impedance significantly larger than rπ, often approximated as Z_in ≈ rπ + (β+1)Re. So degeneration raises the input impedance.

For the output, the collector sees roughly RC in parallel with ro, which sets the output impedance. Emitter degeneration mainly reduces gain through negative feedback and doesn’t drastically change the collector’s impedance path, aside from small, parameter-dependent tweaks. Therefore the output impedance stays around RC || ro, with only modest modulation by transistor parameters.

That’s why the best description is that input is about rπ without degeneration, increases with degeneration due to feedback, and output is ≈ RC || ro, modified by transistor parameters.