Which, if any, of the following proposed voltage thresholds yields a
valid buffer with positive noise margins?
A. VOL = 1.5V VIL = 1.6V VIH = 1.7V VOH = 1.8V
B. VOL = 0.7V VIL = 0.9V VIH = 1.1V VOH = 1.3V
C. VOL = 0.1V VIL = 0.3V VIH = 1.7V VOH = 1.9V
D. VOL = 0.5V VIL = 0.5V VIH = 1.5V VOH = 1.5V
Choice A would allow two "low" inputs (e.g., 1.6V) to generate a "high" output
(1.6 * 1.6 = 2.56 which is above VOH) => not okay.
Choice B would allow two "high" inputs (e.g., 1.1V) to generate an underdetermined
output (1.1 * 1.1 = 1.21 which is above VOL but below VOH)
=> not okay.
Choice C has positive noise margins and the allowed input voltages
result in the desired behavior => okay
Choice D doesn't have positive noise margins => not okay.