TTL or Transistor-Transistor Logic is a type of digital circuit that is made from BJT or bipolar junction transistor along with resistors. Both the amplifying function and the logic gating function are carried out through transistors, thus the name transistor-transistor logic.

TTL is used for many applications like industrial controls, computers consumer electronics, test equipment, synthesizers and more. The TTL designation is also used in some places to imply ‘compatible logic levels’ even if they are not directly associated with transistor transistor logic circuits.

James Buie invented the Transistor Transistor Logic in 1961 and the first Transistor-Transistor Logic devices were made in 1963 in Sylvania. These devices were called the “Sylvania Universal High-Level Logic family” and were used within controls for the US Phoenix missile. In 1964 Texas Instruments produced ICs of 5400 series and later on, the 7400 series which made the Transistor-transistor Logic devices popular amongst electronic system designers. The 7400 series went on to become the industry standard. Many companies like AMD, Motorola, Intel, Fairchild, Siemens, National Semiconductor made compatible parts.

The TTL circuits were low cost which made them highly practical for using digital techniques in tasks which were earlier done through analog methods. One of the first computers that was built, in 1971, made use of the transistor transistor logic instead of a microprocessor chip which at that point of time was not available. With time incremental improvements in power consumption and speed were made, and the last popular series was the 74AS/ALS Advanced Schottky was made available in 1985.