Transistor-Transistor Logic (TTL)
In transistor-transistor logic, the logic gates are constructed around the transistors. It was first introduced in 1964. TTL uses bipolar transistors to construct its integrated circuits. There have been different versions of TTL:
- Standard TTL.
- High speed TTL.
- Low power TTL.
- Schhottky TTL.
We will only discuss the basic TTL. All the TTL families above have three configurations namely:
- Totem-Pole output.
- Open collector output.
- Tristate output.
The input sections of TTL consists of a phase splitter transistor and an input transistor.The input transistor conducts when the emitter-base junction becomes forward biased.
TTL has become the standard logic circuit in many application for a number of years. TTL greatly decreases the manufacturing costs because multiple emitters can be added in the input so no extra space is needed and a multiple input gate can be constructed easily. The output circuit has also been modified in recent years and the the configuration is called "Totem pole". A commercial IC package of TTL includes three three-input gates, four two-input gates, or two four-input gates. The structure of the IC always remains the same.
Emitter Coupled Logic (ECL)
In Emitter coupled logic, the transistors are prevented from going into deep saturation so that there are no storage delays. This logic is used in applications with high speed environment. ECL is considered to be one of the best because there is a very low propagation delay. In fact, it is the fastest bi-polar circuit available today. ECL was first introduced in 1962. This logic family bypasses TTL in terms of speed. ECL is also a non-saturated logic. The logic levels for ECL are normally -0.9V for high logic and -1.6 for low logic. The design of ECL consists of termination resistors which allows the signals to propagate with very low reflection.
Complementary Metal Oxcide Semiconductor Logic (CMOS)
CMOS is known for its low power consumption and high fan-out. The transistors inside the CMOS are made from an NMOS transistor and PMOS transistor. To realize the logical functions, both P-type and N-type transistors are used. It is currently being used in microprocessor technology and Application Specific Integrated Circuits. There is no power dissipation in CMOS. It is also considered to be one of the most reliable logic family today.