Voltage observed at an unused TTL input (floating input behavior) If a standard TTL input is left unused (floating), what approximate voltage is typically measured at that input due to internal biasing?

Introduction / Context:
Unlike CMOS, TTL inputs have internal biasing that tends to pull a floating input to a pseudo-HIGH level. Understanding the typical floating voltage helps diagnose unexpected behavior and reinforces why unused TTL inputs should not be left unconnected in robust designs.

Given Data / Assumptions:

• Technology: standard TTL with multi-emitter input transistor structures.
• Floating inputs are susceptible to noise and can draw extra current.
• The question asks for typical measured voltage at a floating TTL input.

Concept / Approach:

Due to the base-emitter junctions and internal bias paths, a floating TTL input often sits around approximately 1.4–1.8 V. This is neither a clean LOW nor a solid HIGH by CMOS standards, but in TTL it often reads as HIGH. However, relying on this is poor practice because noise can toggle the input and current consumption can increase.

Step-by-Step Solution:

Verification / Alternative check:

Bench measurements on standard TTL families confirm floating inputs hover near ~1.5 V and can be noisy; datasheets and app notes recommend defined connections.

Why Other Options Are Wrong:

Wide ranges (0–5 V or 0.8–5 V) are vague and not diagnostic; 0–1.8 V centers too low for typical TTL biasing.

Common Pitfalls:

Assuming floating is acceptable; for reliability, tie unused inputs HIGH (AND/NAND) or LOW (OR/NOR) via small resistors or directly as recommended.

Final Answer:

1.4 to 1.8 V.