Why is Floating Input High in TTL Logic Circuits

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SUMMARY

Floating inputs in TTL (Transistor-Transistor Logic) circuits, such as the 7400 series, are interpreted as HIGH due to the internal design of the NPN transistor. In these circuits, the base of the transistor is connected to Vcc (typically 5V) through a resistor, which keeps the transistor off when the input is floating or connected to 5V. Grounding the input turns the transistor on by connecting the emitter to 0V. This design avoids issues associated with floating inputs in CMOS technology, where such conditions can lead to unpredictable behavior.

PREREQUISITES
  • Understanding of TTL logic circuits
  • Familiarity with NPN transistor operation
  • Knowledge of resistor-capacitor (RC) circuits
  • Basic electronics schematic reading skills
NEXT STEPS
  • Research the internal structure of the 7400 TTL logic gate
  • Study the differences between TTL and CMOS logic families
  • Learn about input impedance and its effects on circuit behavior
  • Examine practical applications of TTL circuits in modern electronics
USEFUL FOR

Electronics engineers, students studying digital logic design, and hobbyists working with TTL circuits will benefit from this discussion.

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Why floating input is considered as HIGH in TTL based logic ckts ?
 
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Long long ago, a TTL input (for a simple example the 7400) was the emitter of an NPN transistor.

The base of said transistor was pulled up to Vcc (typically 5V in those days) by a resistor inside the chip.

Grounding the input connected the emitter to 0V and turned the transistor on.

Leaving the input floating or connecting it to 5V (usually via a resistor) turned the transistor off.

You can find the schematic of a TTL gate in old datasheets.

Since the input was relatively low resistance, there were none of the problems of CMOS inputs where allowing the input to float is a definite no no...
 

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