Introduction / Context:
MOSFET-based logic (e.g., CMOS) dominates digital design. One bedrock reason is the extremely high input impedance of MOS gates, which draw negligible DC current, enabling low static power and large fan-in without loading previous stages significantly.
Given Data / Assumptions:
- MOSFET gate terminal is insulated (oxide), producing very small DC input currents.
- BJTs require base current to establish conduction, producing finite input impedance.
- We focus on intrinsic device-level advantages, not specific process optimizations.
Concept / Approach:
High input impedance means inputs do not significantly load their drivers at DC. This property underpins CMOS’s near-zero static power (neglecting leakage) and compatibility with high fan-out scenarios, subject to capacitive load considerations for AC switching.
Step-by-Step Solution:
Consider DC behavior: MOS gates draw near-zero steady current → high input impedance.Contrast with BJT base: Requires base current → lower input impedance.Therefore, “high input impedance” is the key, universal advantage.
Verification / Alternative check:
Datasheets show CMOS input currents in nanoamps or less, vastly lower than TTL.
Why Other Options Are Wrong:
higher switching speed / reduced propagation delay: Sometimes true versus TTL, but not universally; depends on process and load.low input impedance: Opposite of MOSFET behavior.
Common Pitfalls:
Ignoring capacitive input effects; high impedance does not mean zero AC load.Assuming MOSFETs are always faster; speed depends on many factors.
Final Answer:
high input impedance
Discussion & Comments