Ecl

Emitter-Coupled Logic (ECL) is a high-speed family of digital logic circuits that uses differential transistor pairs biased by a constant current source. Rather than relying on transistor saturation, ECL performs logic by steering current through matched transistors, which yields fast switching and small propagation delays. ECL circuits are typically powered from a negative supply, historically around -5.2 V, though lower-voltage variants exist. The signal swing is small and the design relies on differential signaling, with outputs that sit near the negative rail. This arrangement provides speed and timing predictability when proper termination is used.

In operation, a differential pair shares a constant emitter current. The input difference directs more current into one transistor than the other, producing a corresponding change at the output node. Logic levels are defined by current states rather than strict rail voltages, so interfacing with other families often requires level shifting or translation (for example, through PECL interfaces to CMOS or TTL).

Advantages of ECL include high speed, tight timing margins, and good noise performance in differential signaling. Disadvantages include relatively high static power consumption and the need for a negative supply and bias networks, which complicate power distribution and interfacing. As a result, ECL is less common for general-purpose logic compared with TTL or CMOS, though it remains favored in some high-speed or signal-critical applications.

History and usage: ECL originated in the 1960s with IBM and was widely used in mainframes, minicomputers, and other high-speed circuitry. Variants such as ECL II and ECL III were developed to improve performance. Today, ECL persists in niche high-speed backplanes and signaling standards, and modern descendants like PECL (positive ECL) provide interfaces toward CMOS/TTL systems.