GATE CS 1998 Q1.15 — Combinational Circuits

In standard Transistor-Transistor Logic (TTL) circuits:

• The typical supply voltage is $V_{CC} = 5\text{ V}$.
• For various TTL subfamilies (such as Low-power Schottky TTL / 74LS), the minimum guaranteed output voltage for a logic high state ($V_{OH}$) under specified load conditions is typically around $2.7\text{ V}$ to $2.8\text{ V}$.
• Therefore, a reliable logic 1 output level is represented by any voltage between this minimum high-level threshold (approx $2.8\text{ V}$) and the supply voltage $V_{CC}$.

Thus, the threshold level for logic 1 in the TTL family is any voltage below $V_{CC}$ but above $2.8\text{ V}$.