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?
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AEach CMOS input adds propagation time which limits the number of loads.
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BAt a critical frequency the gate delivers only 70% of normal output voltage, halving power.
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CMore loads require more frequent servicing, reducing per-gate service rate.
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DThe 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.
Answer
Correct 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.
Explanation
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:
Model each input as capacitance C_in; total load C_total = N * C_in + C_trace.Driver current must support dV/dt across C_total at target edge rate.As f increases, transitions per second rise → higher average and peak current demands.Result: reduce N (fan-out) at higher frequencies to maintain timing and levels.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.