Relative speed — 74LS TTL vs 74HC/HCT (High-Speed CMOS) Approximately how much faster are 74HC/HCT high-speed CMOS families compared to 74LS TTL under typical conditions?

Introduction / Context:
When migrating designs from TTL to CMOS, engineers often compare propagation delays to estimate clock-rate headroom. The 74HC/HCT families were introduced to provide CMOS power advantages with speeds competitive to or exceeding LS TTL, especially in favorable loading conditions.

Given Data / Assumptions:

• 74LS TTL offers modest delays (often on the order of 8–15 ns per gate, depending on device and load).
• 74HC/HCT can achieve much lower delays under light loading due to CMOS drive and low capacitance.
• The question asks for an approximate speed increase factor under typical conditions.

Concept / Approach:

A rule-of-thumb used in many educational contexts is that high-speed CMOS (HC/HCT) can be roughly an order of magnitude faster than older LS TTL in favorable conditions, though actual ratios vary by device, voltage, temperature, and load. HCT further ensures TTL-compatible input thresholds while retaining CMOS speed and power advantages.

Step-by-Step Solution:

Verification / Alternative check:

Consult representative datasheets; specific parts may show smaller or larger improvements, but an order-of-magnitude figure is commonly used as a teaching reference.

Why Other Options Are Wrong:

“5” understates the improvement in many cases; “50” and “100” are typically unrealistic for general-purpose gates without special processes.

Common Pitfalls:

Assuming the ratio is constant across all loads; heavy capacitive loading can erode CMOS speed advantage. Always verify with the exact device and conditions.

Final Answer:

10