Emitter Coupled Logic

Emitter Coupled Logic (ECL), sometimes referred to as Current Mode Logic, is an extremely high-speed digital technology. ECL has a propagation time of 0.5 - 2 ns, which is much faster than TTL. However, its power dissipation is three to 10 times higher than that of TTL.

ECL owes its performance to the transistors never being in saturation, the input/output voltages having a small swing (0.8 V), the input impedance being high and the output resistance being low; as a result, the transistors change states quickly, gate delays are low, and the fanout capability is high.

The output logic of ECL, much like that of TTL, varies from a LOW state to a HIGH state. However, the voltage levels of these states differ between ECL and TTL.

The output logic swing of ECL gates varies from a LOW state of -1.75 volts to a HIGH state of -0.9 volts with respect to ground. The following table is an illustration of when positive logic is used while referring to a logic "0" or "1".

Some common terms used when referring to ECL circuits:

V_EE - Negative power, which is typically -5.2 volts.

V_BB - Switching threshold, which is typically -1.29 volts.

V_TT - Termination voltage, which is typically -2.0 volts.

V_CC - Ground, on most ECL circuits.