In electronics, emitter-coupled logic (ECL) is a high-speed integrated circuit bipolar transistor logic family. ECL uses an overdriven bipolar junction transistor (BJT) differential amplifier with single-ended input and limited emitter current to avoid the saturated (fully on) region of operation and its slow turn-off behavior.As the current is steered between two legs of an emitter-coupled pair, ECL is sometimes called current-steering logic (CSL),current-mode logic (CML)or current-switch emitter-follower (CSEF) logic.