In a BJT common-emitter amplifier with emitter degeneration, how does adding an emitter resistor Re affect AC gain and linearity?

• A. Increases negative feedback, reducing gain and improving linearity; the AC gain approximately Rc/(re' + (β+1)Re)
• B. Decreases negative feedback, increasing gain and reducing linearity
• C. Increases gain and linearity
• D. Has no effect on gain

Answer: (A)

Adding emitter degeneration introduces local negative feedback. As signals try to raise the emitter current, Re drops more voltage, which partially opposes the base-emitter drive and keeps changes in collector current from being as large. In small-signal form, the AC gain of a common-emitter stage with emitter degeneration becomes roughly Av ≈ -Rc / (re' + (β+1)Re). The (β+1)Re term is the emitter resistance reflected back into the input, amplified by the transistor’s current gain. So increasing Re lowers the magnitude of the gain, but it makes the stage more linear because the feedback damps nonlinearities in the transistor’s Vbe–Ic relationship and stabilizes operating conditions. Without Re, the gain is larger (≈ -Rc/re') but the stage is less linear.