What is a Logic Family?

Logic families are different types of technologies being used to build different
logic gates. Logic gates are digital circuits that perform basic logic operations
like AND, OR, NOT, NAND, and NOR. In other words, it is a group of
compatible ICs with the same logic levels and supply voltages fabricated for
performing various logical functions.
Various factors are responsible for choosing which logic families can be used
for the given specific scenarios which include switching speed, fan-out
capabilities, power consumption, etc.
Classification of Logic Families
Logic families can be broadly categorized as per the following diagram:

Unipolar Logic Families

Unipolar means having a single type of charge carrier. This logic family uses
transistors that have either electrons or holes(not both) as charge carriers.
The most commonly used technology is Metal-Oxide-Semiconductor(MOS).
A common example is Complementary MOS (CMOS) logic.
NMOS (N-channel Metal Oxide Semiconductor)
NMOS transistors conduct when a positive charge is applied to the gate
terminal. In NMOS, electrons are the majority carriers. When a high voltage
is applied to the gate, the conduction starts. A negative supply stops the
conduction and thus switches OFF the device. These are generally
considered faster to pMOS as the charge carriers here are electrons which
are twice as fast as holes.
PMOS (P-channel Metal Oxide Semiconductor Battery-Powered)
PMOS transistors conduct when a negative charge is applied to the gate
terminal.
Holes are the majority of charge carriers here. That means the high voltage
supplied to the gate turns the device OFF. Similarly, a low voltage starts the
conduction and in turn, switches ON the device. They are more immune to
noise fluctuations compared to NMOS devices.
CMOS(Complementary MOS)
In CMOS technology, both n-type and p-type transistors are used to design
logical functions. The same signal which turns ON one transistor is used to
turn OFF the other transistor. These characteristics of CMOS make it
compatible for designing logic devices using only simple switching elements,
without the need for a pull-up resistor.
Characteristics of CMOS
Characteristics of CMOS are listed below :
 Low power consumption - CMOS circuit consumes very low power,
making them ideal for battery-powered devices.
 Low cost - The CMOS fabrication process is relatively simpler compared
to other semiconductor technologies.
 High reliability and noise immunity - They are considered to have a
high noise margin and thus are good for circuits that require high
tolerance to noise.
 Limited Voltage Swing - They have a low voltage range of operation,
making them less suitable for high voltage operations.
 Process Variation - The CMOS fabrication process is highly dependent
on process conditions, leading to variations that can affect the
performance and reliability of the final product.
 Vulnerability to electrostatic discharge - CMOS is greatly affected by
electrostatic discharge leading to permanent device damage on exposure.
Bipolar Logic Families
In bipolar devices, the conduction happens due to both charge carriers -
electrons and holes. Bipolar logic families use semiconductor diodes and
bipolar junction transistors as the basic building blocks of logic circuits.
There are further classifications of the bipolar logic family in two types
Saturated
In this logic, the bipolar junction transistors(BJTs) used are operated in
saturated regions. This means that both the emitter-base and collector-base
junctions are forward-biased, allowing maximum current flow through the
transistor.
Characteristics of Saturated Logic Families
Characteristics of Saturated Logic Families are listed below :
 Can allow relatively higher current through transistors
 Very fast state switching
 Higher power consumption than non-saturated logic families.
 Better noise immunity
 Examples include Transistor-Transistor Logic (TTL), Diode Transistor
Logic (DTL), and Resistor Transistor Logic (RTL). TTL is the most popular
category in this classification.
Transistor-Transistor Logic (TTL)
Transistor-transistor logic (TTL) is a digital logic family employing bipolar
junction transistors (BJTs) to uphold logic states and facilitate switching
operations. Introduced in 1961 by James L. Buie of TRW, TTL remains
prevalent in various electronic devices and systems. Renowned for its
remarkable performance and adaptability, TTL finds widespread application
in logic gates, memory circuits, and microprocessors.
Characteristics of TTL
Characteristics of TTL are listed below :
 Logic Voltage Levels: TTL logic inputs are classified as logical high
when they fall between 2V and 5V, and logical low when within the range
of 0V to 0.8V.
 Propagation Delay: TTL stands out for having the minimal propagation
delay among digital integrated circuits (ICs).
 Power Dissipation: A standard TTL device consumes approximately
10mW of power.
 Noise Margin: TTL boasts a noise margin of about 0.4V
 Fan Out: Typically, TTL exhibits a fan-out capability of 10.
 Supply Voltage: TTL necessitates a supply voltage ranging between
4.75 V and 5.25 V.
 Speed: TTL is renowned for its rapid switching speed.
 Compatibility: TTL devices are compatible with other TTL devices.

Non-saturated
In non-saturated bipolar logic, the bipolar junction transistors (BJTs) are
operated in the active or linear region and not in the saturation region. In
other words, the collector-base junction is reverse-biased, limiting the current
flow through the transistor.
Characteristics of Non-Saturated Logic Families
Characteristics of non-saturated Logic Families are listed below :
 Comparatively lower current flow than the saturated logic family's
transistors
 Slower switching speed
 Lower power consumption
 Examples include Emitter Coupled Logic(ECL) and Schottky TTL.
Emitter Coupled Logic (ECL) family
Emitter-coupled logic (ECL) is a bipolar transistor logic family that is
considered to be the fastest logic available. It was invented in 1956 at IBM.
The key to reducing propagation delay in a bipolar logic family is to prevent a
gate’s transistors from saturating, we learned how Schottky diodes prevent
saturation in TTL gates.
ECL is used in high-performance applications, such as: Clock-distribution
circuits, High-frequency-based applications, Fiber-optic transceiver
interfaces, Ethernet, and ATM (Asynchronous Transfer Mode) networks.
Characteristics of ECL logic family
Characteristics of ECL Logic Families are listed below :
 Power noise: ECL circuits generate relatively little power noise
 Propagation time: The propagation time for ECL can be less than a
nanosecond
 Small voltage swing: ECL achieves its high-speed operation by
employing a relatively small voltage swing and preventing the transistors
from entering the saturation region
 No external inverters: ECL devices operate without the need for any
external inverters to simultaneously create the true and complementary
output of the desired function at the outputs
 Small voltage swing: ECL has a small swing which generally varies with
difference of 0.8V
Comparison of a Logic Family
The following table presents a comprehensive comparison of popular logic
families on various parameters
Advantages and Disadvantages of Different Logic
Families
The following table classifies the major benefits and limitations of the three
most common logic families - Complementary - MOS (CMOS) family,
Transistor-Transistor Logic (TTL) family, and Emitter-Coupled-Logic(ECL)
family.
Advantages of CMOS
 Extremely low power consumption
 High fan-out (~50)
 Can operate in wider temperature ranges(-55 to 125 degree C)
 No static power dissipation. Power is dissipated only for switching
MOSFETs.
 Best Noise Immunity
Disadvantages of CMOS
 Slow speed of operation
 Propagation delay time is around 50ns while this is around 10-12ns in
TTL
Advantages of TTL
 Least susceptible to electrical damage
 Noise immunity is better than ECL but less than CMOS
 Compatible with other logic families
 Lesser propagation delay than CMOS
 Better switching speed
Disadvantages of TTL
 Moderate power consumption.
 Prone to temperature variations
 Large power dissipation
 Poor noise immunity
Advantages of ECL
 Fastest speed
 Lesser temperature interference
Disadvantages of ECL
 Power consumption is higher than TTL and CMOS
 Lower operating voltage
 Very low noise immunity
Applications of the Logic Gate
Given below are the Applications of the Logic gate
Applications of CMOS
 Digital ICs: Microprocessors, Microcontrollers, Memory chips
 Embedded systems: Robotics, Automotive electronics
 Signal Processing: Analog-to-digital converters(ADCs), filters, amplifiers
 Medical devices: MRI scanners, Pacemakers
Applications of TTL
 Legacy systems: Industrial plants having legacy systems implanted
 Testing instruments: Oscilloscopes, Logic analyzers, signal generators
Applications of ECL
 High-speed computing: In legacy mainframe and super-computers
 Telecommunications: High-speed switches, routers and communication
interfaces
 Military and Aerospace: Radars, Missile guidance system

Which is the fastest logic family?

Emitter-coupled logic (ECL) is considered to have the fastest switching
speed. It is a bipolar junction transistor(BJT) and achieves a fast speed by
having lower voltage swings and making transistors not go into saturation
thus having a low storage delay
NAND Gate Using CMOS Technology
The NAND gate can be implemented in CMOS technology by using PMOS and
NMOS transistors. The circuit diagram of a two input NAND gate in CMOS
technology is shown in the following figure −

It consists of two PMOS transistors Q1 and Q2 and two NMOS transistors Q3

and Q4. The PMOS transistors are connected in parallel between the power
supply VDD and the output terminal Y. Similarly, the NMOS transistors are
connected in series between the output terminal Y and the ground terminal
GND.

Now, let us understand the operation of this CMOS NAND gate.

Case 1: When Input A is Low and Input B is Low

In this case, when both inputs A and B are low, the PMOS transistors Q1 and Q2
are ON and the NMOS transistors Q3 and Q4 are OFF. Hence, there is a closed
path between the supply voltage VDD and the output terminal Y.

Thus, the output Y will be connected to the voltage level V DD. Also, there is no
path between the output terminal and the ground terminal as both NMOS
transistors are OFF. Under this condition, the output line will maintain the
voltage level at VDD, which indicates the output High.

Thus, when A = 0 and B = 0, then Y = 1

Case 2: When Input A is Low and Input B is High

In this case, the PMOS transistor Q1 will be ON while the PMOS transistor Q2
will be OFF. The NMOS transistor Q3 will be OFF and the NMOS transistor Q4
will be ON.

For this switching condition of the CMOS transistors, the power supply V DD will
get a path to the output terminal through the PMOS transistor Q1. Since, the
NMOS transistor Q3 and Q4 are connected in series and the NMOS transistor Q3
is OFF. Hence, there is no path between the output terminal and the ground
terminal.

Therefore, the output terminal Y maintain the voltage level at V DD and results in
a High output.

Thus, when A = 0 and B = 1, then Y = 1

Case 3: When Input A is High and Input B is Low

In this case, the PMOS transistor Q1 will be OFF and the PMOS transistor Q2 will
be ON. The NMOS transistor Q3 will be ON and the NMOS transistor Q4 will be
OFF.

Under this switching condition of the CMOS transistors, the output terminal will
connect to the power supply through the PMOS transistor Q2. Since, both NMOS
transistors are connected in series and the NMOS transistor Q4 is OFF. Hence,
there is no path between the output terminal and the ground terminal.

Therefore, the output line will maintain the voltage level at V DD and results in a
High output.

Thus, when A = 1 and B = 0, then Y = 1

Case 4: When Input A is High and Input B is High

In this case, both PMOS transistors Q1 and Q2 will be OFF and both NMOS
transistors will be ON. In this case, there is no path between the output
terminal and the power supply VDD, but there is a direct path between the
output terminal and the ground terminal. This results in a ground voltage level
at the output terminal and produces a Low output.

Hence, when A = 1 and B = 1, then Y = 0

The operation of this CMOS NAND gate is shown in the following truth table −

Inputs Output

A B Y

Low (0) Low (0) High (1)

Low (0) High (1) High (1)

High (1) Low (0) High (1)

High (1) High (1) Low (0)

This is all about NAND gate implementation using CMOS technology and its
operation for different input combinations.

NOR Gate Using CMOS Technology

Similar to CMOS NAND gate, we can also design a NOR gate using PMOS and
NMOS transistors. The circuit diagram of a two input NOR gate using CMOS
technology is shown in the following figure −
This CMOS NOR gate is designed by using two PMOS transistors Q1 and Q2 and
two NMOS transistor Q3 and Q4. Where the PMOS transistors are connected in
series between the supply voltage VDD and the output terminal Y. The NMOS
transistors are connected in parallel between the output terminal Y and the
ground terminal GND.

Now, let us understand how does this CMOS circuit operate as a two input NOR
gate.

Case 1: When Input A is Low and Input B is Low

In this case, both PMOS transistors Q1 and Q2 will be ON and both NMOS
transistors Q3 and Q4 will be OFF.

Under this switching condition of the CMOS transistors, there is a path between
the supply voltage VDD and the output terminal Y through the ON PMOS
transistors. But there is no path between the output terminal Y and the ground
terminal GND. This maintains the output at the voltage level V DD and hence the
output will be High.

Thus, when A = 0 and B = 0, then Y = 1

Case 2: When Input A is Low and Input B is High

In this case, the PMOS transistor Q1 is ON, the PMOS transistor Q2 is OFF, the
NMOS transistor Q3 is OFF, and the NMOS transistor Q4 is ON.

Since, the PMOS transistors Q1 and Q2 are connected in series and the
transistor Q2 is OFF. Thus, there is no path between the power supply V DD and
the output terminal Y. But there is a connection between the output line Y and
the ground terminal GND through the ON NMOS transistor Q4. This sets the
output terminal to ground voltage and makes the output Low.

Therefore, when A = 0 and B = 1, then Y = 0

Case 3: When Input A is High and Input B is Low

In this condition, the PMOS transistor Q1 is OFF, the PMOS transistor Q2 is ON,
the NMOS transistor Q3 is ON, and the NMOS transistor Q4 is OFF.

In this case, there is no closed path between the power supply V DD and the
output line Y due to OFF PMOS transistor Q1. But there is a closed between the
output line Y and the ground terminal GND through the ON NMOS transistor Q3.
 Hence, the output terminal is connected to the ground potential and makes the
output Low.

Thus, when A = 1 and B = 0, then Y = 0

Case 4 – When Input A is High and Input B is High

In this case, both PMOS transistors Q1 and Q2 are OFF and both NMOS
transistors Q3 and Q4 are ON. Under this condition, there is no path between
the supply voltage VDD and the output terminal Y. But there is a closed path
between the output terminal Y and the ground terminal GND. This maintains the
output line at ground voltage level and hence the output will be Low.

Thus, when A = 1 and B = 1, then Y = 0

This complete operation of the CMOS NOR gate can be summarized in the form
of a truth table which is given below.

Inputs Output

A B Y

Low (0) Low (0) High (1)

Low (0) High (1) Low (0)

High (1) Low (0) Low (0)

High (1) High (1) Low (0)

Advantages of NAND and NOR Gates using CMOS Technology

NAND and NOR gates implemented in CMOS technology offer several benefits
over other technologies. Some of the key advantages of CMOS NAND and NOR
gates are listed here −

 CMOS NAND and NOR gates consume relatively low power. This advantage makes
these logic gates well-suited to use in battery powered devices.
 NAND and NOR gates designed using CMOS technology have high immunity
against noise and interference. They can be designed to have a wider range of
operating voltage.
 The CMOS technology offers high-density integration that allows for
implementing a large number of NAND and NOR gates on a single chip. These
gates provide symmetrical output characteristics that allow them to integrate
with different types of digital circuits seamlessly.
 CMOS technology is one of the well-established, mature, and cost-effective
semiconductor manufacturing technology. Hence, the CMOS NAND and NOR
gates are relatively easy to manufacture and cost effective.

Applications of CMOS NAND and NOR Gates

The CMOS NAND and NOR gates are widely used in the following applications
due to their benefits and versatility −

 CMOS NAND and NOR gates are widely used in the logic circuit designs to
perform logical operations.
 In digital systems, the CMOS NAND and NOR gates are used to implement
arithmetic circuits like adders, subtractors, multipliers, etc.
 They are also used in memory units to implement memory cell structures.
 CMOS NAND and NOR gates are also used to design multiplexers and
demultiplexers.
 Some other common applications of CMOS NAND and NOR gates include digital
signal processing, digital timing circuits, analog to digital conversion, digital
communication, etc.
CMOS and TTL Interfaces
To achieve optimum performance in a digital system, devices from more
than one logic family can be used, taking advantages of the superior
characteristics of each family for different parts of the system. For example,
CMOS logic ICs can be used in those parts of the system where low power
dissipation is required, whereas TTL can be used for those portions of the
system which require high speed of operation. Also, some function may be
easily available in TTL and others may be available in CMOS. Therefore, it is
necessary to examine the interface between CMOS and TTL devices.
CMOS and TTL are the two most widely used logic families. Although ICs
belonging to the same logic family have no special interface requirements,
that is, the output of one can directly feed the input of the other, the same
is not true if we have to interconnect digital ICs belonging to different logic
families. Incompatibility of ICs belonging to different families mainly arises
from different voltage levels and current requirements associated with LOW
and HIGH logic states at the inputs and outputs.

The interfacing involves following techniques:

1. Voltage level shifting: adjusting voltage levels to match input

requirements.

2. Pull Up or Pull Down resistors: to ensure proper logic levels

3. Buffers: To safely translate signals

Sr. No. Feature TTL CMOS

1. Logic 1 2V to 5V Close to Vcc ( 3V, 5V ,
12V)
2. Logic 0 0V to 0.8 V Close to 0V
3. Power Consumption Higher Lower
4. Fan out 10 >50
5. Input Impedance Low High

a. Interfacing TTL to CMOS:

5 V is given to TTL and CMOS. The Logic levels of TTL and CMOS are
different. TTL output in high state yields 2.4 Volt which is lower than the
minimum voltage required by CMOS IC which is 3.5 V. For TTL to CMOS
Interfacing, Pull UP resistor is connected which solves the interfacing
problem.
b. Interfacing CMOS to TTL:
A CMOS IC can easily drive any low power Schottky TTL IC directly. But to interface standard TTL IC,
Buffer is provided in between CMOS and TTL ICs.