Rise time is defined as the time it takes for the output to go from what fraction to what fraction of its maximum value?

• A. A 0.0 to 1.0
• B. B 0.5 to 0.95
• C. C 0.1 to 0.9
• D. D 0.2 to 0.8

Answer: (C)

Rise time measures how fast an output edge develops. It is defined as the time required for the output to rise from 10% to 90% of its final (steady-state) value. This focus on the central portion of the transition captures the edge speed while avoiding the small initial offset near zero and the slow settling tail near the final value. For many systems, especially approximately first-order ones, the 10–90% interval relates to the time constant and provides a practical sense of bandwidth. For example, in an RC-like response, the rise time is on the order of a couple of time constants. Other ranges would not characterize the edge the same way: from 0% to 100% includes the entire approach to the final value and the tail, 50% to 95% skips the initial part of the rise and focuses on a later portion, and 20% to 80% omits both the very start and the final approach. Hence the standard definition uses 10% to 90%.