Digital logic families

Name-Sahil Kale

Roll No-13

 Digital Logic Families

Logic Families

A logic family is a collection of different integrated circuit chips that have similar input, output, and internal circuit characteristics, but they perform different logic gate functions such as AND, OR, NOT, etc. The idea is that different logic gate functions, when fabricated in the form of an integrated circuit with the same approach, or which belongs to the same logic family, will have identical electrical characteristics (electrically compatible with each other). These families may vary by speed, power consumption, cost, voltage and current levels.

Classification of the digital logic family

According to the components used, there are different types of logic families. Some of the logic families include Resistor-Transistor logic(RTL), Diode-Transistor logic(DTL), Transistor-transistor logic(TTL), Emitter coupled logic(ECL), PMOS, NMOS, and CMOS circuits.

Bipolar Logic Family:

Bipolar Logic FamilyIt mainly uses bipolar devices like diodes, transistors in addition to passive elements like resistors and capacitors.There are two kinds of operations in bipolar integrated circuits: Saturated Bipolar Logic family and Non-saturated Bipolar Logic family.

Saturated Bipolar Logic Families

In this family the transistors used in ICs are driven into saturation. 

1. Diode logic (DL)

2. Resistor Transistor Logic (RTL)

3. Diode Transistor Logic (DTL)

4. Integrated Injection Logic (IIL or I2L)

5. Transistor Transistor Logic (TTL)

6. High Threshold Logic (HTL)

Non-saturated Bipolar Logic Families

 In this family the transistors used in IC is not driven into saturation.

1. Schottky TTL

2. Emitter Coupled Logic (ECL)

Unipolar Logic Family:

Unipolar logic family consists of Metal Oxide Semiconductor (MOS) logic families. It mainly uses Unipolar devices like MOSFETs in addition to passive elements like resistors and capacitors. These logic families have the advantages of high speed and lower power consumption than Bipolar families. They are:

1. P - type MOS (PMOS) Logic

2. N - type MOS (NMOS) logic

3. Complementary MOS (CMOS) logic

4. Bipolar MOS (BiMOS) logic

5. Bipolar CMOS (BiCMOS) logic

Features of Logic:

1. TTL - Transistor-Transistor Logic: Standard logic family; used for the longest time.

2. ECL - Emitter Coupled Logic: Suitable for systems requiring high-speed operations.

3. MOS - Metal Oxide Semiconductor Logic: Suitable for systems with high component density.

4. CMOS - Complementary Metal Oxide Semiconductor Logic: Suitable for systems with low power consumption (VLSI circuits). Gradually becomes the dominant logic family.

Characteristics of the digital logic family:

(i) Propagation Delay

It is the time interval between the application of the input pulse and the occurrence of the output. It is an important characteristic of the digital logic family.

If the propagation delay is less, then the speed at which the IC operates will be faster.

Let THL is the propagation delay when the output changes from logic 0 to 1 and TLH is the delay when the output changes from logic 1 to 0. The maximum value of THL and TLH is considered as the propagation delay for that logic gate.

(ii) Power Dissipation

It is the amount of power that the digital circuit dissipates. The power dissipated is determined by the average current, that is drawn from the supply voltage.

The average current is the average value of the current at LOW gate output(logic ‘o’) and the current at HIGH gate output (logic ‘1’).

Fan in and Fan out

Fan-in refers to the number of inputs in a digital logic gate family. For the example given in the figure below, the EX-OR gate has three inputs. So fan-in for the given EX-OR gate is 3.

Fan-out

 Fan out refers to the number of inputs that is driven by the output of another logic gates. For example, the following circuit has an EX-OR gate, which drives 4 NOT gates. So fan-out of EX-OR gate is 4.

CMOS v/s TTL

There are two different types of computer chips: CMOS and TTL. CMOS stands for complementary metal-oxide-semiconductor. TTL stands for transistor-transistor logic. They’re both oftentimes used to make microchips. However, they’re both different and serve different functions. To learn more about the difference between CMOS and TTL, we must understand its functioning.

The difference between CMOS and TTL is that CMOS (complementary metal-oxide-semiconductor) is an electronic component in the logic family. CMOS is used in computers and various other electronic devices. TTL (transistor-transistor logic) is a type of digital logic in the logic family. TTL is also used in computers and various other electronic devices.

CMOS, Or complementary metal oxide semiconductor, uses PMOS and NMOS types of field-effect transistors to handle input and output. They are commonly used in NAND and NOR gates and are economically efficient, and have a higher clock rate making them consume more power than TTL or transistor-transistor logic units.

TTL and Transistor-Transistor Logic Units are made up of BJTs Or bipolar junction transistors that commonly have multiple inputs and outputs. They have a slightly lower density of logic gates and usually are found with NAND gates. However, they consume less power and hence are used in several devices.

Conclusion

 Digital logic families are the backbone of modern electronics, enabling the design and implementation of complex digital systems. Each family has its unique characteristics, making it suitable for specific applications. Understanding these families and their attributes is essential for engineers, hobbyists, and anyone working with digital circuits. As technology continues to advance, new logic families and variations emerge, catering to the ever-evolving needs of the electronics industry. Whether you're designing a cutting-edge microprocessor or tinkering with a hobby project, the choice of the right logic family will significantly impact the performance and efficiency of your digital circuit.