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In electronics, emitter coupled logic, or ECL, is a logic family in which current is steered through bipolar transistors to compute logical functions. ECL is sometimes called 'current mode logic'.

The chief characteristic of ECL is that the transistors are always in the active region and can thus change state very rapidly, allowing ECL circuits to operate at very high speed. Its major disadvantage is that the transistors are continuously drawing current, which means the circuits require a lot of power. Most of this power is wasted as heat.

TTL and related families use transistors as digital switches where transistors are either cut off or saturated, depending on the state of the circuit. 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 collector configuration). The propagation time for this arrangement can be less than a nanosecond, making it the fastest logic family through 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.

The ECL circuits available on the open market usually operated with negative power supplies (-5.2 volts), and logic levels incompatible with other families. This meant that interoperation between ECL and other logic families, such as the popular TTL family, required additional interface circuits. The fact that the high and low logic levels are relatively close meant that ECL suffers from small noise margins, which can be troublesome.

At least one manufacturer, IBM, made ECL circuits for use in the manufacturer's own products; the power supplies were substantially different from those used in the open market.^[1]

The drawbacks associated with ECL have meant that it has been used mainly when high performance is a vital requirement. Other families (particularly advanced CMOS variants) have replaced ECL in many applications, even mainframe computers. However, some experts predict increasing use of ECL in the future, particularly in conjunction with more widespread adoption of advanced semiconductors such as GaAs, which has always been seen as the semiconductor of the future, but cannot be produced as cheap/clean as Si and is toxic.

Older high-end mainframe computers, such as the Enterprise System/9000 members of IBM's ESA/390 computer family, used ECL^[1]; current IBM mainframes use CMOS.

The equivalent of emitter coupled logic made out of FETs is called source coupled FET logic (SCFL).

1. ^ ^{a b} A. E. Barish et al. (1992). "Improved performance of IBM Enterprise System/9000 bipolar logic chips". IBM J. of Research and Development 36 (5): 829–834. 

• resistor-transistor logic (RTL)
• diode-transistor logic (DTL)
• transistor-transistor logic (TTL)
• Complementary Metal Oxide Semiconductor (CMOS)
• Positive Emitter Coupled Logic (PECL)
• Low Voltage Positive Emitter Coupled Logic (LVPECL)
• Integrated injection logic (I2L)

• Emitter-Coupled Logic
• Motorola MECL logic family datasheets, 1963

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