Saturated-switching in TTL: Evaluate — “The 74XX series TTL operates using saturated switching, where many conducting transistors enter saturation.”
-
ACorrect
-
BIncorrect
-
CCorrect only for 74HC family
-
DCorrect only with Schottky clamps present
-
EOnly true at Vcc below 4.5 V
Answer
Correct Answer: Correct
Explanation
Introduction / Context:Standard TTL (74XX) logic uses multi-emitter input transistors and relies on saturated switching in its internal stages. Saturation stores charge and limits speed; this motivated Schottky TTL variants to avoid deep saturation and improve performance.
Given Data / Assumptions:
- “74XX” refers to the original TTL family, not CMOS (74HC/HCT).
- Saturation occurs when base–collector junction becomes forward-biased.
- Stored charge in saturation increases turn-off delay.
Concept / Approach:In basic TTL, output transistors drive into saturation to achieve low VOL. The trade-off is speed versus simplicity and power. Variants like 74S/74LS include Schottky clamps to prevent deep saturation while preserving compatibility.
Step-by-Step Solution:
Recall that standard TTL outputs use a saturated NPN to pull output LOW.Recognize that such saturated operation is a defining characteristic of classic TTL.Thus, the statement accurately describes 74XX TTL operation.Verification / Alternative check:Timing diagrams and internal schematics in datasheets show saturated output devices for 74XX; Schottky families are advertised as “non-saturating” improvements.
Why Other Options Are Wrong:
- Incorrect / HC/HCT: CMOS families (HC/HCT) are not TTL transistors; Schottky-only caveats miss the point that 74S/74LS specifically avoid deep saturation, unlike 74XX.
Common Pitfalls:Confusing all 74-series as TTL (some are CMOS); assuming Schottky clamps are present in classic 74XX (they are not).
Final Answer:Correct