Introduction / Context:
Propagation delay is the time from an input transition to the corresponding valid output transition. In TTL devices, this delay arises from the finite switching dynamics of transistors and the charging/discharging of internal and load capacitances.
Given Data / Assumptions:
- TTL uses multi-transistor stages (input transistor network, phase splitter, totem-pole output).
- Each transition involves storage and removal of charge in junctions.
- External loading also adds capacitance that must be driven.
Concept / Approach:
“Slow switching speed” is a lay description of the physical causes: transistor storage time, finite carrier recombination, and RC time constants. All contribute to non-zero rise/fall times and internal stage delays, producing an overall propagation delay specification (e.g., 10 ns to tens of ns depending on the family).
Step-by-Step Solution:
Identify propagation delay sources → device internal switching + load effects.Recognize that faster processes (e.g., Schottky clamps) cut storage time, reducing delay.Therefore, propagation delay is indeed due to finite (not instantaneous) switching → statement is correct.
Verification / Alternative check:
Datasheet timing confirms lower delays for faster variants (e.g., 74S, 74AS) due to improved switching elements.
Why Other Options Are Wrong:
Incorrect: Ignores well-understood transistor switching physics.Only correct for LS families / high temperature: Delay varies with family and temperature but the underlying cause remains finite switching.
Common Pitfalls:
Treating propagation delay as unrelated to switching; they are directly linked.Assuming logic-level compatibility affects intrinsic delay; it primarily affects interfacing.
Final Answer:
Correct
Discussion & Comments