ECL (Single-Ended or Differential)

Emitter Coupled Logic (ECL) was introduced as an alternative to TTL logic because it is better suited for high speed data transmission. Emitter Coupled Logic circuits use transistors to steer current through gates that compute logical functions. Because the transistors are always in the active region, they can change state very rapidly, so ECL circuits can operate at very high speeds.

ECL suffers from two disadvantages. First, ECL requires relatively high currents to operate. Secondly, ECL relies on a negative power supply for operation. This can cause problems when interfacing to positive-supply-based devices residing in the rest of the system. However, being referenced to ground can offer noise advantages