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TTL input threshold recognition: For a standard transistor–transistor logic (TTL) gate, what is the minimum input voltage that is guaranteed to be recognized as a logic HIGH (valid 1)?

Difficulty: Easy

Correct Answer: 2.0 V

Explanation:


Introduction / Context:
Digital interfacing requires knowing the guaranteed logic thresholds. TTL families define input high and low thresholds so that designers can compute noise margins and safe interfacing to other families such as CMOS (HC/HCT).



Given Data / Assumptions:

  • Standard TTL input thresholds: VIL(max) ≈ 0.8 V, VIH(min) ≈ 2.0 V.
  • Standard TTL output levels: VOL(max) ≈ 0.4 V, VOH(min) ≈ 2.4 V.
  • Supply voltage is 5 V nominal.


Concept / Approach:
The minimum input voltage recognized as HIGH is VIH(min). Values below this may not be reliably interpreted as a 1. Values at or above VIH(min) must be recognized as HIGH by any compliant TTL input, allowing some positive noise margin from the VOH(min) of a driving TTL gate.



Step-by-Step Solution:

List TTL thresholds: VIH(min) = 2.0 V, VIL(max) = 0.8 V.Compare to options: 2.0 V is the minimum guaranteed HIGH threshold.Therefore the correct answer is 2.0 V.


Verification / Alternative check:

TTL datasheets for 74xx families consistently specify VIH(min) ≈ 2.0 V.


Why Other Options Are Wrong:

2.4 V: This is VOH(min) for TTL outputs, not the input threshold.0.8 V: That is VIL(max), the maximum LOW input voltage.5.0 V: That is the supply, not the threshold.


Common Pitfalls:

Confusing VOH(min) (output) with VIH(min) (input).Assuming thresholds are symmetric around half the supply; TTL is not symmetric.


Final Answer:

2.0 V
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