CMOS vs. TTL — key advantage in typical digital designs What is the principal advantage of using CMOS logic families compared with TTL logic families under similar operating conditions?

Introduction / Context:
Choosing a logic family affects speed, power, noise margins, and system cost. Complementary MOS (CMOS) and TTL have different transistor technologies and therefore different strengths. This item targets the most widely cited advantage of CMOS.

Given Data / Assumptions:

• Standard low-power operation at moderate frequencies.
• Supply voltage appropriate to the family (e.g., 3.3 V or 5 V).
• Static versus dynamic power characteristics are considered.

Concept / Approach:

CMOS draws negligible static power when gates are not switching (idealized case: only leakage). Power consumption is primarily dynamic: P ≈ C_load * V^2 * f. TTL, by contrast, has higher static currents due to bipolar junction operation, leading to greater standby and overall power usage at comparable clock rates.

Step-by-Step Solution:

Verification / Alternative check:

Datasheets for modern HC/HCT/AC CMOS show microampere quiescent currents, versus milliampere-level ICC for classic TTL at 5 V, confirming substantial power savings in CMOS designs.

Why Other Options Are Wrong:

• It switches faster: not universally true; modern CMOS can be fast, but speed depends on subfamily.
• It is larger: CMOS ICs are not inherently larger physically.
• Cost: depends on family and vendor; not the universal advantage.
• No decoupling capacitors: all digital ICs need decoupling for transient currents.

Common Pitfalls:

• Assuming CMOS is always faster than TTL; check timing specs for the specific subfamily.
• Ignoring input protection—CMOS is ESD-sensitive and requires proper handling.

Final Answer:

It uses less power.