For a BJT common-emitter amplifier biased with a voltage divider, explain why emitter degeneration is used.

• A. Emitter degeneration provides negative feedback, stabilizes bias, increases linearity, and reduces gain sensitivity to β variations.
• B. Emitter degeneration increases gain without affecting stability.
• C. Emitter degeneration eliminates bias variations.
• D. Emitter degeneration reduces linearity.

Answer: (A)

Emitter degeneration uses a resistor in the emitter path to create negative feedback. When the transistor current tends to rise, more current flows through the emitter resistor, raising the emitter voltage. That higher emitter voltage reduces the effective base–emitter voltage and limits the increase in collector current. If the current tends to fall, the emitter voltage drops, allowing more current to flow again. This self-correcting loop stabilizes the operating point, making it less sensitive to variations in transistor beta and to temperature changes.

Because part of the input signal is effectively fed back through the emitter, the stage’s transconductance is reduced, which improves linearity of the gain and makes the bias point more predictable. The base voltage set by the divider would otherwise be more strongly perturbed by base current and device variations, but the emitter feedback dampens those effects, keeping the bias stable. In many designs, a bypass capacitor across the emitter resistor is added for AC signals to recover gain while preserving DC stability.