Emitter coupled logic
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In electronics, emitter coupled logic (or ECL) (sometimes called 'current mode logic') is a logic family in which current is steered through transistors to compute logical functions.
ECL's chief characteristic is that the transistors are always in the active region and they can change state very rapidly, so ECL circuits can operate at very high speed . Its major disadvantage is that the transistors are continually drawing current, which means the circuits require high power, and thus generate large amounts of waste heat.
Explanation
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.Characteristics
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.
Power supplies and logic levels
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 these levels are used outside a chip.
Usage
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, which has always been the semiconductor of the future, but cannot be produced as cheap/clean as Si and is toxic.
One field of application for ECL is some mainframe computers, such as the Enterprise System/9000 members of IBM's ESA/390 computer family.
The equivalent of emitter coupled logic made out of FETs is called source coupled FET logic (SCFL).
References
See also
- 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)
External links
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