Transistor Transistor Logic or TTLThe full form of TTL is Transistor Transistor Logic. This is a logic family which is mainly build up of NPN transistors, PN junction diodes and diffused resistors. The basic building block of this logic family is NAND gate and there are various subfamilies of this logic gate those are standard TTL, advanced Schottky TTL, schottky TTL, low power TTL, high power TTL, fast TTL etc. now to know about this family in a more descriptive way we will discuss the internal structure and characteristic parameters of some of its subfamilies.
Standard TTLThe above figure shows the internal structure and characteristics of a standard TTL NAND gate. The NAND gate of it is a quad two input type. And it has four circuits of 5400/740. In plain ways the circuit of this type of TTL operates as follows. The Q1 showed in the figure is a two emitter NPN transistor.
This type is NAND gate is analogous to two transistors whose base and emitter terminals are joined together. The diodes named as D2 and D3 are used to limit the input voltages which are negative in nature.
Low Power TTLThis is a subfamily under the main family. This is named so because lower power consumption and dissipation is achieved. Though the speed at which the operation is done is somewhat reduced. The above figure is of a low power TTL which is made using AND gates. The NAND gate used in this is of 74L00 or 54L00 type and is of quad two input type. The construction of this type of TTL is almost similar to that of standard TTL except the resistance which is of a higher value. For this increased value of the resistance the power dissipation of the circuit is lowered.