Effect of removing the emitter bypass capacitor in a common-emitter (CE) BJT amplifier Consider a CE amplifier that originally uses a bypass capacitor across the emitter resistor to maximize midband gain. If this bypass capacitor is removed (leaving the emitter resistor unbypassed), how do the midband voltage gain and the small-signal input resistance change?

Introduction / Context:
The emitter bypass capacitor in a common-emitter (CE) amplifier shunts the emitter resistor for AC signals, reducing local feedback and boosting midband voltage gain. Removing this capacitor reintroduces emitter degeneration, significantly affecting both gain and input resistance. Understanding this trade-off is key to designing stable, linear, and predictable amplifiers.

Given Data / Assumptions:

• Single-transistor CE amplifier with emitter resistor RE and bypass capacitor CE.
• Midband operation (coupling capacitors act as AC shorts; transistor in forward-active region).
• Small-signal analysis; bias conditions maintained.

Concept / Approach:
With CE installed, the small-signal emitter node is close to AC ground, minimizing degeneration and maximizing gain: Av ≈ −gm * (RC || RL) * rπ / (rπ + RSIG) in one common form. Removing CE raises the effective emitter impedance (approximately (1 + β) * RE seen at the base), increasing negative feedback. This reduces transconductance-to-load conversion and raises the input resistance since the base sees a larger resistance reflected from the emitter.

Step-by-Step Solution:

Verification / Alternative check:
Compare measurements: you will observe a smaller midband gain and a larger input resistance after removing CE. Frequency response often flattens (more stable gain) and distortion decreases due to feedback.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming the bypass only affects low frequency. In fact, at midband, bypassing is what removes the emitter resistor’s AC impact; removing it reintroduces feedback across the band.

Final Answer:
decrease, increase