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Transistor–transistor logic (TTL) is a logic family built from bipolar junction transistors.Its name signifies that transistors perform both the logic function (the first "transistor") and the amplifying function (the second "transistor"), as opposed to earlier resistor–transistor logic (RTL) and diode–transistor logic (DTL).
Transistor–transistor logic uses bipolar transistors to form its integrated circuits. [12] TTL has changed significantly over the years, with newer versions replacing the older types. Since the transistors of a standard TTL gate are saturated switches, minority carrier storage time in each junction limits the switching speed of the device.
Standard transistor–transistor logic (TTL) uses transistors as saturated switches. A saturated transistor is turned on hard, which means that it has a lot more base drive than it needs for the collector current it is drawing. The extra base drive creates a stored charge in the base of the transistor.
A digital use of a push–pull configuration is the output of TTL and related families. The upper transistor is functioning as an active pull-up, in linear mode, while the lower transistor works digitally. For this reason they are not capable of sourcing as much current as they can sink (typically 20 times less).
7400 series parts were constructed using bipolar junction transistors (BJT), forming what is referred to as transistor–transistor logic or TTL. Newer series, more or less compatible in function and logic level with the original parts, use CMOS technology or a combination of the two ( BiCMOS ).
Diode–transistor logic improved the fan-out up to about 7, and reduced the power. Some DTL designs used two power supplies with alternating layers of NPN and PNP transistors to increase the fan-out. Transistor–transistor logic (TTL) was a great improvement over these. In early devices, fan-out improved to 10, and later variations reliably ...
The bipolar junction transistor, the first type of transistor to be mass-produced, is a combination of two junction diodes and is formed of either a thin layer of p-type semiconductor sandwiched between two n-type semiconductors (an n–p–n transistor), or a thin layer of n-type semiconductor sandwiched between two p-type semiconductors (a p ...
Generally, a TTL output does not rise high enough to be reliably recognized as a logic 1 by a CMOS input, especially if it is only connected to a high-input-impedance CMOS input that does not source significant current. This problem was solved by the invention of the 74HCT family of devices that uses CMOS technology but TTL input logic levels.