Emitter coupled logic

In electronics, emitter coupled logic (or ECL) is a design
which uses transistors to steer current through gates which
compute logical functions. By comparison, TTL
and related families use transistors as digital switches,
where transistors are either cut off or saturated, depending on the
state of the circuit. This distinction explains ECL's chief
advantage: that because the transistors are always in the active
region, they can change state very rapidly, so ECL circuits can
operate at very high speed; and also its major disadvantage: the
transistors are continually drawing current, which means the circuits
require high power, and thus generate large amounts of waste heat.

ECL gates use differential amplifier configurations at the input
stage. A bias configuration supplies a constant voltage at the
midrange of the low and high logic levels to the differential
amplifier, so that the appropriate logical function of the input
voltages will control the amplifier and the base of the output
transistor. The propagation time for this arrangement can be less
than a nanosecond.

Other noteworthy characteristics of the ECL family include the fact
that the large current requirement is approximately constant, and does
not depend significantly on the state of the circuit. This means that
ECL circuits generate relatively little power noise, unlike many other
logic types which typically draw far more current when switching than
quiescent, for which power noise can become problematic. ECL circuits
usually operate with negative power supplies, and use logic levels incompatible
with other families, which means that interoperation between ECL and
other designs is difficult. The fact that the high and low logic
levels are relatively close mean that ECL suffers from small noise
margins, which can be troublesome in some circumstances.

The drawbacks associated with ECL have meant that it has been used
mainly when high performance is a vital requirement, and other
families (particularly advanced CMOS variants) have been gradually
taking over ECL use in some applications. However, some experts
predict increasing use of ECL in the future, particularly in
conjunction with more widespread adoption of advanced semiconductors
such as GaAs.

History

The first "family" of digital logic integrated circuits was
the MECL family introduced by Motorola in 1962. This was
succeeded by the MECL II and MECL III families, then the industry-standard
10K ECL and 100K ECL families.