Side-by-side comparison
| Parameter | TTL | CMOS Logic Families |
|---|---|---|
| Technology | Bipolar junction transistors (NPN, diodes) | Complementary MOSFET pairs (PMOS + NMOS) |
| Supply voltage | Fixed 5 V (74LS, 74ALS) | 2–6 V (74HC); 1.65–5.5 V (74LVC) |
| Quiescent power per gate | 2 mW (74LS); 1 mW (74ALS) | 0.001 mW (74HC at DC); scales with frequency |
| Propagation delay (typical) | 9 ns (74LS); 4 ns (74ALS) | 22 ns (74HC at 5 V); 7 ns (74ACT) |
| Noise margin | VIL max = 0.8 V, VIH min = 2.0 V; NM ≈ 400 mV | VIL = 30% Vcc, VIH = 70% Vcc; NM ≈ 1.4 V at 5 V |
| Fan-out | 10 (74LS driving 74LS) | ~50 (CMOS input draws < 1 µA) |
| Input impedance | Low; ~1.6 mA sink per input | Very high; gate oxide, < 1 µA leakage |
| Output drive | Totem-pole output; IOL = 8 mA, IOH = 0.4 mA | IOL = 4 mA, IOH = 4 mA (74HC); symmetrical |
| Susceptibility to static | Low; BJT inputs tolerate ESD better | High; gate oxide breakdown; needs ESD handling |
| Typical ICs | 74LS00, 74LS74, 74ALS138 | 74HC00, 74HCT00, 74LVC1G04 |
Key differences
TTL logic uses bipolar transistors that draw significant base current — a single 74LS input sinks 1.6 mA, so fan-out is limited to 10. CMOS inputs are MOSFET gates drawing under 1 µA, giving fan-out above 50. CMOS power consumption is proportional to switching frequency — P=CV²f — so at 10 MHz with C=15 pF and Vcc=5 V, each gate dissipates only 3.75 mW, while a 74LS gate draws 2 mW regardless of whether it is switching. Noise margin is another CMOS advantage: at 5 V, CMOS requires VIH > 3.5 V versus 2.0 V for TTL, making CMOS far more immune to noise. The 74HCT series bridges the gap — CMOS power with TTL-compatible input thresholds for mixed-family designs.
When to use TTL
Use TTL (74LS or 74ALS) when operating at 5 V with strict timing requirements and moderate fan-out — for example, a 74ALS138 3-to-8 decoder in a 5 V bus interface where propagation delay must stay under 5 ns.
When to use CMOS Logic Families
Use CMOS (74HC or 74LVC) when power consumption, supply flexibility, or noise immunity drives the design — for example, a 74LVC logic gate operating from a 3.3 V lithium battery in a portable IoT sensor node where quiescent current must stay below 100 µA.
Recommendation
For any new digital design, choose CMOS — 74HC or 74LVC. The power advantage is overwhelming and noise margins are superior. Use TTL only when interfacing with legacy 74LS systems or when the absolute minimum propagation delay at 5 V is required. Know the 74HCT family for mixed TTL-CMOS interfacing.
Exam tip: Examiners test noise margin calculations: for 74LS, NMH = VOH(min) − VIH(min) = 2.7 − 2.0 = 0.7 V and NML = VIL(max) − VOL(max) = 0.8 − 0.5 = 0.3 V — practise deriving both from the datasheet specifications.
Interview tip: Interviewers at digital hardware companies ask you to explain why CMOS power consumption increases with frequency while TTL does not, and to calculate power for a CMOS gate at a given frequency using P = CV²f.