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Transistor-transistor logic (TTL) is a class of integrated circuits which maintain logic states and achieve switching with the help of bipolar transistors. One of the prominent features of transistor-transistor logic signals is the ability of the inputs of the gate rise to the logical "1" if left unconnected. Transistor-transistor logic is one of the reasons that integrated circuits are so widely used, as they are less expensive, more reliable and faster than resistor-transistor logic and diode-transistor logic.
A transistor-transistor logic device makes use of transistors with multiple emitters in gates having multiple inputs. There are different sub-categories or families for transistor-transistor logic, such as:
One of the biggest benefits of using transistor-transistor logic is the relative easiness in interfacing different circuits and the ability to produce complex logic functions. This is mainly due to good noise margins as well as guaranteed voltage levels. Transistor-transistor logic has good “fan in” feature, meaning the number of input signals that can be accepted by an input. Transistor-transistor logic is largely immune to damage from static electricity discharges, unlike CMOS, and are also relatively inexpensive compared to CMOS.
One major disadvantage of transistor-transistor logic is its high current consumption. The heavy current demands of transistor-transistor logic can lead to improper functioning due to switching of output states. Even with different transistor-transistor logic versions that are less current consuming, they are all still competitive to CMOS.
With the advent of CMOS, some applications using TTL have been supplanted by CMOS. However, transistor-transistor logic is still used in applications as they are fairly robust and the gates are relatively inexpensive.