Schottky TTL behavior: Do Schottky TTL logic gates reduce saturation delay by preventing the transistors from entering deep saturation (thereby speeding up turn-off)?
-
AValid — Schottky clamps prevent deep saturation and shorten storage delay.
-
BInvalid — Schottky diodes only lower noise margins with no speed benefit.
-
CInvalid — the speed-up comes only from higher supply voltage, not device structure.
-
DInvalid — Schottky TTL operates slower than standard TTL due to extra junctions.
Answer
Correct Answer: Valid — Schottky clamps prevent deep saturation and shorten storage delay.
Explanation
Introduction / Context:Standard TTL devices can suffer from storage delay when a transistor saturates. Schottky TTL adds Schottky diodes to clamp the base–collector junction, limiting saturation and cutting turn-off time. This question verifies understanding of that speed mechanism.
Given Data / Assumptions:
- Schottky barrier diode between base and collector of key transistors.
- Aim is to reduce storage time, not merely dynamic switching loss.
- Supply voltage is the same as standard TTL (typically 5 V).
Concept / Approach:When a BJT saturates, excess charge must be removed from the base region during turn-off, creating storage delay. A Schottky clamp conducts before the transistor would enter deep saturation, holding VBC small and preventing charge storage. Result: faster transitions and smaller propagation delay.
Step-by-Step Solution:
Recognize that deep saturation creates storage charge.Schottky clamp turns on at a low forward voltage, shunting base current and preventing saturation.With less stored charge, turn-off delay and tPLH/tPHL are reduced.Verification / Alternative check:Datasheets for 74S/74LS families show lower propagation delays than 74xx standard TTL. The improvement is attributed to Schottky clamping and other optimizations.
Why Other Options Are Wrong:
- No speed benefit (option b) contradicts measured delays.
- Higher VCC (option c) is not the cause; device structure is.
- Slower due to “extra junctions” (option d) misrepresents Schottky’s role; it actually avoids stored charge.
Common Pitfalls:Confusing Schottky TTL with ECL or assuming speed comes only from power consumption increases.
Final Answer:Valid — Schottky TTL reduces saturation delay via clamping.