CMOS fan-out behavior: Evaluate — “The fan-out of CMOS gates is frequency dependent.”

Introduction / Context:
CMOS inputs are high-impedance and draw negligible DC current, which suggests very high DC fan-out. However, at practical operating frequencies, input capacitance and trace capacitance demand dynamic charging/discharging current. This behavior makes effective fan-out frequency dependent.

Given Data / Assumptions:

• CMOS inputs present capacitance (Cin), not significant DC current.
• Output stage has finite drive (Ron, Iout).
• Rise/fall times degrade as capacitive load increases with more loads or higher frequency.

Concept / Approach:
As frequency increases, the product of load capacitance and edge rate requirements forces more current, slowing transitions and possibly violating timing/noise margins. Therefore, the number of inputs that a CMOS output can reliably drive at a given frequency (i.e., effective fan-out) decreases as frequency rises.

Step-by-Step Solution:

Verification / Alternative check:
Vendor datasheets specify maximum capacitive loads for given rise/fall times and recommended loading guidelines versus clock frequency.

Why Other Options Are Wrong:

• Incorrect / frequency-limited statements: The dependence exists across a wide frequency range; it is not restricted to a narrow band.

Common Pitfalls:
Equating DC fan-out (near infinite) with practical AC fan-out; ignoring trace capacitance and routing topology; overlooking that multiple receivers may switch simultaneously.

Final Answer:
Correct