Why CMOS fan-out depends on frequency CMOS outputs can drive many inputs at low speed, but the allowable fan-out decreases as frequency rises. What is the fundamental reason for this frequency dependence?

Introduction / Context:
Fan-out quantifies how many inputs a single output can reliably drive. Unlike TTL, CMOS inputs draw almost no DC current; however, they present capacitance that must be charged and discharged every transition. This turns fan-out into a frequency-dependent limit tied to dynamic current and timing margins.

Given Data / Assumptions:

• CMOS inputs behave mainly as capacitors.
• Output edges must charge/discharge the sum of all input capacitances plus wiring capacitance.
• Higher frequency means more transitions per second and increased dynamic load.

Concept / Approach:
Dynamic current roughly follows I ≈ C_total * V * f * activity. For a fixed driver, as the number of loads increases, C_total increases, slowing edges and increasing current. At higher f, the same C_total demands faster charge/discharge, risking degraded VOH/VOL or timing violations. Therefore, practical fan-out must fall as frequency climbs to preserve edge integrity.

Step-by-Step Solution:

Verification / Alternative check:
Check datasheets for dynamic power and AC drive limits; IBIS simulations confirm edge-rate degradation with added capacitive loads.

Why Other Options Are Wrong:

• A/C: Fan-out is not about “servicing” or additive propagation times per input.
• B: The 70% statement is unrelated to CMOS fan-out physics.

Common Pitfalls:
Ignoring trace capacitance; long routes can dominate over the device input capacitance.

Final Answer:
The input gates of the FETs are predominantly capacitive, and as signal frequency increases the capacitive loading also increases, limiting the number of loads that may be attached.