Signal coupling function — which capacitor role passes AC while blocking DC between two stages?

Introduction / Context:
Capacitors in analog circuits serve different roles depending on where they are placed. A common use is to transfer the AC component of a signal from one stage to another while preventing any DC bias from shifting the next stage’s operating point. This use case has a specific name.

Given Data / Assumptions:

• Two amplifier/processing stages with different DC bias points.
• A capacitor placed in series between stages.
• Linear small-signal operation around bias.

Concept / Approach:
A coupling capacitor is placed in series with the signal path. Its reactance is low at signal frequencies (so AC passes) and effectively infinite at DC (so DC is blocked). Bypass/decoupling capacitors are typically shunt elements to ground that stabilize supply rails or provide AC ground at certain nodes; they do not sit in series as a conduit for AC between stages.

Step-by-Step Solution:

Verification / Alternative check:
Basic small-signal models show that the series capacitor and the input resistance of the next stage form a high-pass network that sets the lower cutoff frequency f_c = 1/(2piR_in*C).

Why Other Options Are Wrong:

• Filter: Non-specific; many filters exist and may not block DC by default.
• Bypass / Decoupling: Typically shunt to ground for supply or local AC stabilization, not series AC transfer.

Common Pitfalls:
Confusing “bypass” (to ground) with “coupling” (in series path); overlooking that decoupling ≈ bypass on power rails.

Final Answer:
Coupling capacitor.