CMOS driving TTL — matching current and logic-level requirements When CMOS logic circuits must drive TTL logic circuits, different current requirements often cause interface issues. Which practical addition is commonly used to overcome this mismatch and ensure reliable logic levels?

Introduction / Context:
Interfacing different logic families—specifically CMOS outputs driving TTL inputs—can fail if the driver cannot source/sink the input currents that the receiver expects. This question focuses on the practical hardware addition that restores noise margin and guarantees valid HIGH/LOW levels when CMOS must drive TTL loads.

Given Data / Assumptions:

• CMOS outputs (especially older 4000-series) have limited source/sink current compared to TTL requirements.
• TTL inputs draw more input current (notably IIL when low) and have VIH(min) near 2.0 V to 2.4 V for standard TTL.
• Goal: achieve reliable logic-level compatibility and drive strength without redesigning the entire system.

Concept / Approach:
Adding a buffer stage tailored to the source family solves both current drive and edge-rate issues. A CMOS buffer or inverting buffer (e.g., 4049/4050-type or HC/HCT/ACT family buffers) provides higher fan-out and sharper transitions, ensuring that TTL VIH/VIL thresholds are met under load. If HCT or dedicated “TTL-compatible” CMOS buffers are used, their input thresholds are designed to accept TTL levels while their outputs can drive multiple TTL inputs.

Step-by-Step Solution:

Verification / Alternative check:
Consult buffer datasheets for IOH/IOL and VOH/VOL specs at desired fan-out. Lab measurements with an oscilloscope will show restored edge integrity and margins.

Why Other Options Are Wrong:

• A and C: “Bilateral switches” are analog pass devices, not ideal for robust digital level translation or current drive.
• B: A TTL tristate buffer may work in some cases, but the question generalizes CMOS→TTL; a CMOS buffer solution is the standard recommendation.

Common Pitfalls:
Ignoring input thresholds (TTL vs. CMOS) and relying only on pull-up resistors; this can degrade edges and noise immunity at higher speeds.

Final Answer:
a CMOS buffer or inverting buffer