Bipolar digital ICs
Digital ICs consist of gates which perform logical operations at high speeds. There are many different technologies, each having its own unique characteristics. Digital-logic technology may use bipolar transistors or metal oxide semiconductor (MOS) devices.

TTL
In the transistor-transistor logic (TTL), arrays of bipolar transistors, some having multiple emitters, operate on direct current pulses. This technology has many variants, some of which date back to around the year 1970. The hallmark of TTL is the immunity to noise pulses. A simple TTL gate is shown in the figure is given below. The transistors are completely cut off, or else completely saturated. This is reason why TTL is not much affected by external noise distractions.

ECL
Another bipolar transistor logic form is called as emitter-coupled logic (ECL). In ECL, the transistors are not operated at saturation, as they are with TTL. This increases speed of operation of ECL compared with TTL. But noise pulses have a greater effect in ECL, as unsaturated transistors amplify as well as switch signals. The schematic of the figure given shows a simple ECL gate