Side-by-side comparison
| Parameter | ECL | TTL Logic Families |
|---|---|---|
| Switching technology | Differential pair; transistors never saturate (current-mode) | Saturating bipolar transistors (totem-pole output) |
| Propagation delay | 0.1–2 ns (MC10101, MC100E series) | 9 ns (74LS); 4 ns (74ALS) |
| Maximum clock frequency | Up to 1–3 GHz for MC100E ECL | ~50 MHz (74LS); ~200 MHz (74ALS) |
| Power consumption | Very high; 25–50 mW per gate (constant, not switching-dependent) | 2–10 mW per gate (74LS/74ALS) |
| Supply voltage | −5.2 V (PECL uses +3.3 V or +5 V) | +5 V fixed |
| Output levels | VOH ≈ −0.9 V, VOL ≈ −1.75 V (NECL); small swing of ~0.8 V | VOH ≈ 3.4 V, VOL ≈ 0.2 V; swing of ~3 V |
| Noise margin | ~200 mV (small swing makes ECL noise-sensitive) | 300–400 mV (74LS) |
| Wired-OR capability | Yes; open-emitter outputs support wired-OR | No; totem-pole outputs cannot be wire-OR'd |
| Termination required | Yes; 50 Ω transmission line termination mandatory | No; unterminated short wires acceptable at low speed |
| Typical ICs | MC10101, MC100E151, MC10H125 (ECL/TTL translator) | 74LS00, 74ALS374, 74F logic family |
Key differences
ECL avoids transistor saturation by using a differential pair with an emitter-coupled topology — the transistors switch between partial conduction states, never reaching hard saturation, so there is no stored charge to clear on the falling edge. This gives ECL gates like the MC10101 a propagation delay of 1 ns versus 9 ns for the 74LS00. The cost is constant high power: each ECL gate draws 25 mW regardless of switching activity, making a 10,000-gate ECL chip dissipate 250 W. The small output swing of 0.8 V also means ECL is noise-sensitive — 200 mV noise margin versus 400 mV for TTL — and all ECL lines must be properly terminated in 50 Ω to prevent reflections at GHz speeds.
When to use ECL
Use ECL when propagation delay must be under 2 ns or clock frequencies exceed 200 MHz — for example, an MC100E151 ECL D flip-flop clocked at 1 GHz in the clock distribution network of a high-speed serialiser/deserialiser (SerDes) IC.
When to use TTL Logic Families
Use TTL when operating at standard digital logic speeds below 50 MHz and power consumption or cost must be controlled — for example, a 74ALS374 octal D flip-flop in a 10 MHz 8-bit data bus interface between a microprocessor and peripheral ICs.
Recommendation
For any design running below 100 MHz, choose TTL or preferably CMOS — ECL's power consumption and negative supply complexity are not justified. Choose ECL only when propagation delay under 2 ns is a hard requirement that CMOS cannot meet. In practice, modern GHz designs use LVDS or CML instead of classical ECL.
Exam tip: Examiners ask why ECL is faster than TTL — the complete answer is non-saturation of transistors, small output voltage swing, and differential current switching; write all three for full marks.
Interview tip: Interviewers at high-speed hardware companies ask you to explain the wired-OR capability of ECL and why TTL totem-pole outputs cannot be wired together — the BJT pull-up in TTL would short against a low-driving output, causing excessive current.