Emitter coupled logic
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In electronics, emitter coupled logic (or ECL) is a design which uses transistors to steer current through gates which compute logical functions (as does every logic family). By comparison, TTL and related families use transistors as digital switches (ECL is also digital), 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 (this output transistor is used in common emitter configuration?). The propagation time for this arrangement can be less than a nanosecond, making it the fastest logic family thru several years.
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. In an ALU - where a lot of switching occurs - ECL can draw lower mean current than CMOS.
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 if one wants to use these levels outside a chip.
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, wich has always been the semiconductor of the future, but cannot be produced as cheap/clean as Si and is toxic.
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.