TTL vs CMOS Comparison

When you pick an IC off a shelf, choosing between TTL and CMOS determines power budget, speed, noise tolerance, and whether your circuit even works with the rest of the board. Engineers made this trade-off daily through the 1980s and 1990s, and GATE still tests it rigorously.

Core Concept

TTL gates use bipolar junction transistors (BJTs). Current flows through the base of the input transistor in the logic HIGH state, meaning TTL inputs draw significant current (20–40 µA per input for 74LS). The totem-pole output stage has a small resistor between VCC and ground that creates quiescent current even in steady state. A 74LS00 gate dissipates about 2 mW when idle. An entire 74LS-based board can easily consume several watts just sitting still.

CMOS uses MOSFETs with insulated gate oxide. Gate input current is below 1 µA. In steady state, no DC path exists between VDD and GND, so static power is essentially zero. The 74HC00 quad NAND has tpd ≈ 8 ns at 5V and requires only nanoamps of quiescent current. This is why virtually all modern digital ICs — microcontrollers, FPGAs, SoCs — are built in CMOS.

Speed-wise, the older 74S (Schottky TTL) reached tpd ≈ 3 ns but consumed 20 mW per gate. The 74AC and 74ACT CMOS families match or beat TTL speed: 74AC00 achieves tpd ≈ 5 ns at near-zero static power. The 74HCT family bridges both worlds — CMOS internals with TTL-compatible input thresholds (VIH min = 2.0V), making direct substitution of 74LS ICs possible.

Boolean Expression

Both families implement the same Boolean functions. The difference is electrical, not logical. Noise margin NMH = VOH(min) − VIH(min) and NML = VIL(max) − VOL(max). For 74LS TTL: NMH = 2.7V − 2.0V = 0.4V, NML = 0.8V − 0.4V = 0.4V. For 74HC at 5V: NMH ≈ 4.4V − 3.5V = 0.9V, NML ≈ 1.0V − 0.1V = 0.9V. CMOS has more than twice the noise margin of standard TTL.

Given:
Compare noise margins for 74LS00 (TTL) and 74HC00 (CMOS @ 5V)

Formula / Rule:
NMH = VOH(min) - VIH(min)
NML = VIL(max) - VOL(max)

Step by step — 74LS00 (TTL):
VOH(min) = 2.7V,  VIH(min) = 2.0V  → NMH = 2.7 - 2.0 = 0.7V
VIL(max) = 0.8V,  VOL(max) = 0.4V  → NML = 0.8 - 0.4 = 0.4V
Worst-case noise margin = min(NMH, NML) = 0.4V

Step by step — 74HC00 (CMOS @ 5V):
VOH(min) = 4.4V,  VIH(min) = 3.5V  → NMH = 4.4 - 3.5 = 0.9V
VIL(max) = 1.0V,  VOL(max) = 0.1V  → NML = 1.0 - 0.1 = 0.9V
Worst-case noise margin = 0.9V

Final Answer:
CMOS noise margin (0.9V) is 2.25x better than TTL (0.4V) at 5V

Exam Tip: GATE frequently tests interfacing rules. A TTL output driving a CMOS input can fail because TTL VOH(min) = 2.7V but CMOS VIH(min) = 3.5V at 5V — a 0.8V gap. The fix is to use 74HCT (VIH min = 2.0V) or add a pull-up resistor. Never assume TTL directly drives standard CMOS without checking voltage levels. Also note that fan-out calculations differ: TTL fan-out uses current (IOH/IIH), while CMOS fan-out is limited by capacitive loading and timing, not DC current.

Key Properties

• TTL (74LS): VCC = 5V ± 5%, static power ≈ 2 mW/gate, tpd ≈ 10 ns, fan-out = 10
• CMOS (74HC): VDD = 2–6V, static power ≈ 0, tpd ≈ 7 ns at 5V, fan-out ≥ 10
• TTL VIH min = 2.0V, VIL max = 0.8V — narrower forbidden zone than CMOS
• CMOS at 5V: VIH min = 3.5V, VIL max = 1.0V — wider forbidden zone, higher noise immunity
• 74HCT: CMOS family with TTL-compatible inputs (VIH min = 2.0V) — direct 74LS replacement
• 74AC/74ACT: advanced CMOS, tpd ≈ 5 ns, near-zero static power — fastest standard logic
• Interfacing TTL→CMOS at 5V requires pull-up resistor or 74HCT; CMOS→TTL is generally safe

Quick Revision

• TTL uses BJTs; CMOS uses MOSFETs — this drives all electrical differences
• TTL static power ≈ 2 mW/gate; CMOS static power ≈ 0 (major reason CMOS dominates)
• TTL noise margin ≈ 0.4V; CMOS noise margin ≈ 0.9V at 5V
• TTL input draws 40 µA; CMOS input draws < 1 µA — affects fan-out calculation method
• 74HCT bridges TTL and CMOS: CMOS internals, TTL input thresholds
• 74AC/ACT is the fastest standard logic at ≈ 5 ns with CMOS power levels
• Both families implement identical Boolean logic — the difference is purely electrical
• Exam trap: assuming TTL output can directly drive standard CMOS input at 5V — it cannot without a pull-up or 74HCT