AC grounding in amplifier design In small-signal amplifiers, which capacitor is intentionally placed to create an AC ground at a node (while leaving DC bias unchanged)?

Introduction / Context:
Many amplifier stages use capacitors for different roles: coupling signals between stages, stabilizing bias, and shaping frequency response. The term “AC ground” refers to a point that is held at nearly zero AC potential, even though it may sit at a nonzero DC voltage. Creating an AC ground strategically can increase gain and reduce noise.

Given Data / Assumptions:

• Small-signal amplifier with bias networks.
• Capacitors large enough in value to behave as low reactance at the signal frequency of interest.
• DC operating point must remain undisturbed.

Concept / Approach:
A bypass capacitor connects a node (often the emitter/source or a bias divider midpoint) to ground for AC. Since capacitors block DC, the biasing resistors still set the DC voltage. For AC, the low reactance path shunts signal variations to ground, reducing degeneration and increasing stage gain. Coupling capacitors, by contrast, pass AC between stages but do not by themselves create an AC ground at a fixed node.

Step-by-Step Solution:
Identify node where AC variations are unwanted (e.g., emitter resistor).Place a sufficiently large capacitor from that node to ground.At signal frequency, XC ≈ 1/(2πfC) is small → node held near AC ground.DC bias unaffected due to capacitor’s open-circuit behavior at DC.

Verification / Alternative check:
Frequency sweep shows gain rising when the emitter/source resistor is bypassed at midband, with a roll-off at low frequencies where XC increases and bypassing becomes ineffective.

Why Other Options Are Wrong:
Coupling capacitor: transfers AC between stages, not an AC ground creator.DC open / AC open: descriptive phrases, not specific capacitor types in this context.

Common Pitfalls:
Using too small a bypass value, resulting in partial bypass and unintended low-frequency attenuation.

Final Answer:
bypass capacitor