Transistor-Transistor Logic, abv. TTL

Transistor–Transistor Logic (TTL) is a class of digital circuits built from bipolar junction transistors (BJT), and resistors. It is called transistor–transistor logic because both the logic gating function and the amplifying function are performed by transistors. It is notable for being a widespread integrated circuit (IC) family used in many applications such as computers, industrial controls, test equipment and instrumentation, consumer electronics, synthesizers, etc. Because of the wide use of this logic family, signal inputs and outputs of electronic equipment may be called "TTL" inputs or outputs, signifying compatibility with the voltage levels used.

TTL integrated circuits are examples of small-scale to large-scale integration. Each "chip" contains the equivalent of a few dozen to a few hundred transistors, contrasting with early very-large-scale integration (VLSI) devices that had the equivalent of up to 10,000 transistors, and modern microprocessors that are equivalent to tens of millions of transistors.

The fundamental switching action of a TTL gate is based on a multiple-emitter input transistor. The active operation of this input transistor removes stored charge from the output stage transistors more rapidly than a comparable DTL gate, making TTL much faster in switching. A small amount of current must be drawn from a TTL input to ensure proper logic levels. The total current drawn must be within the capacities of the preceding stage, which limits the number of nodes that can be connected (the fanout).

All standardized common TTL circuits operate with a 5 volt power supply. A TTL signal is defined as "low" or L when between 0V and 0.8V with respect to the ground terminal, and "high" or H when between 2.4V and 5V.

Definition from Wikipedia