Electronic Science

1

Module-1:

Logic Families – Characteristics and Types



Table of Content
1.1 Introduction

1.2 Logic families

1.3 Positive and Negative logic

1.4 Types of logic families

1.5 Characteristics of logic families

1.6 Evolution of logic families

1.7 Classification of logic families

1.8 Summary



Learning Outcome:

After completing this module, you will be able to

1. Understand need of logic digital ICs

2. Understand significance of logic families

3. Understand characteristics of logic families

4. Identify different types of logic families

5. Know about evolution of different logic families




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1. Logic families

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1.1 Introduction

The first logic circuit was developed using discrete circuit components. Using advance
techniques, these complex circuits can be miniaturized and produced on a small piece of
semiconductor material like silicon. Such a circuit is called integrated circuit (IC). Now-a-days,
all the digital circuits are available in IC form. While producing digital ICs, different circuit
configurations and manufacturing technologies are used. This results into a specific logic family.
Each logic family designed in this way has identical electrical characteristics such as supply
voltage range, speed of operation, power dissipation, noise margin etc.

In this module, we discuss significance and types of logic families. The positive and negative
logic and its significance are also discussed. In addition to this, different characteristics which
are the key parameters in deciding the logic family for any circuit design are discussed in detail.
The module is concluded with explanation of the brief history of the logic family in terms of
discrete logic circuits.

1.2 Significance of Logic families

Almost all electronic gadgets make use of different digital systems for their operation. All the
digital systems use some kind of digital ICs. For the sake of simplicity in design and
compatibility in constructing any complex digital system, all digital circuits (ICs) used in the
design process should be from same logic family.

For the expansion of the system, it is necessary to connect different logic circuits together. In
order to connect the output of one logic circuit to the input of another logic circuit, one must
have circuit with similar characteristics. If the electrical IO characteristics of these logic circuits
are not similar, there is a need to design an interfacing circuit to maintain the compatibility of
digital logic ICs. This interfacing circuit will match the electrical characteristics of the logic
circuits. This ensures the compatibility for proper operation of the circuit.

This is the key concept behind family for logic circuits. In a logic family, the family members
have similar electrical characteristics. Digital logic circuit has to be designed, considering these

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compatibilities of different logic families in terms of different characteristics and parameters
associated with the families.

1.3 Types of logic families

As explained in previous section, logic families are the logic circuits having identical electrical
parameters. It is a group of compatible ICs with the same logic levels and supply voltages for
performing various logic functions. They are fabricated using a specific circuit configuration
which is referred to as a Logic family. The circuit design of the basic gate of each logic family is
the same.

The logic family is designed by considering the basic electronic components such as resistors,
diodes, transistors, and MOSFET; or combinations of any of these components. Accordingly,
logic families are classified as per the construction of the basic logic circuits. Many different
logic families of digital ICs have been introduced commercially are listed in table 1.

Table 1. Logic families and the components used for construction of logic family

Name of logic family Components used
DL(Diode Logic) Diodes
RTL(Resistor Transistor Logic) Resistors and transistors
DTL(Diode Transistor Logic) Diodes, transistors and resistors
TTL(Transistor Transistor Logic) Transistors and resistors
ECL(Emitter Coupled Logic) Transistors and diodes
PMOS(P channel Metal Oxide Semiconductor P- MOSFETs
Logic)
NMOS(N channel Metal Oxide N- MOSFETs
Semiconductor Logic)
CMOS(Complementary Metal Oxide P –MOSFET and N-MOSFET
Semiconductor Logic)

Logic families are classified according to the principle type of electronic components used in
their circuitry as shown in Figure 1. They are


Digital Electronics
Electronic Science
1. Logic families