The various logic families are Transistor-Transistor logic, CMOS logic, ECL logic families.

TTL [Transistor-Transistor logic]

• BJTs operating in saturated mode.
• Limited switching speed (storage time).

ECL [Emitter-Coupled logic]

• BJTs operating in unsaturated mode (i.e. emitter-follower mode).
• Principle: Current switching [ECL is also sometimes called Current Mode logic].
• Disadvantages:
  
  • Negative supply …