MOSFET input characteristics — consequence of very high input impedance Which statement best describes the practical effect of the high input impedance of MOSFET-based logic inputs?

Introduction / Context:
Metal–Oxide–Semiconductor Field-Effect Transistors (MOSFETs) present extremely high DC input impedance because the gate is insulated by a thin oxide. This property fundamentally differentiates CMOS logic from bipolar TTL, especially with respect to input current, power, and loading effects in digital systems.

Given Data / Assumptions:

• MOSFET gate leakage is tiny in normal operation.
• CMOS input current is near zero except for brief transitions and leakage.
• We are considering typical room-temperature operation and intact gate oxides.

Concept / Approach:
Because essentially no DC current flows into a MOS gate, driving a CMOS input does not impose a static current load. The main dynamic load is capacitive (charging and discharging the gate capacitance), so average input power remains low at modest frequencies. This translates to lower power dissipation and high fan-out potential from a DC perspective.

Step-by-Step Reasoning:

Verification / Alternative check:
CMOS datasheets specify input leakage in nanoamps or picoamps, confirming minimal DC input current compared with TTL in the hundreds of microamps per input for some families.

Why Other Options Are Wrong:

• “Allows faster switching”: MOSFETs can be fast, but high input impedance alone does not guarantee speed; capacitances and drivers dominate.
• “Prevents dense packing”: the opposite—low static power encourages density.
• “Creates low-noise reactions”: noise depends on many factors; not a direct result of high impedance.
• “Requires series resistors”: not mandatory; only used in specific protection/RC cases.

Common Pitfalls:

• Ignoring the capacitive nature of CMOS inputs; while DC is low, fast edges require proper drive strength.

Final Answer:
reduces input current and power dissipation