EVOLUTION OF DIGITAL LOGIC FAMILIES

In today's fast paced world, where everything is available just at a click on our mobiles or Laptops might seem like a Magic...

But,

How do these little wonders work???

What makes them operate so fast???

Might be among the few questions coming to your mind ....

So basically if we give an insight into all these gadgets,it comprises of electronic components placed on one small flat piece of semiconductor that is normally silicon,these are called as Integrated Circuits(IC).These components are place in orderly manner so as to function properly. IC's are made up of logic circuits.Logic circuits came from different logic families.So in a digital system if all the required IC's belong to a particular logic family then there is no problem of compatibility.But if different logic families are brought together then it becomes necessary to ensure compatibility,so for this we must understand different kinds of logic families and make use of best combination to make integrated circuits.

The first logic circuit was developed using discrete circuit components.Using advanced techniques these complex circuit got miniaturized and began to be produced on a semiconductors.

So what exactly are logic families ??

Logic families are a group of  logic circuits and gates that are used in Integrated Circuits.
These logic families are classifies into -
1. Unipolar
2. Bipolar
In unipolar the current flows in one direction.So in these families MOSFET's  are being used and in bipolar families the current flows in two directions so transistors like BJT are used.

Further classification is done as shown below-




Designing of logic families can be done using resistors,transistors,diodes,MOSFET, or as a combination with one another as show below-

NAME OF LOGIC FAMILY	COMPONENT
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 Logic)	P- MOSFETs
NMOS (N channel Metal Oxide Semiconductor Logic)	N- MOSFETs
CMOS (Complementary Metal Oxide Semiconductor Logic)	P –MOSFET and N MOSFET

Out of the logic families mentioned above each of these have their own advantage and disadvantage,but TTL,ECL & CMOS are widely used in many digital circuit design application.
ECL are known for their high speed operations,CMOS which is widely used in VLSI are known for its low power consumption.

How to recognize these IC's ? 
So to avoid the confusion IC's are given name which is combination of few numbers and alphabet.
1.TTL ICs are designated by 5400 and 7400 series.
  The 5400 series has a wide operating range and 7400 series has narrower operating range .
2.ECL IC's are designated by 10000 series.
3.CMOS IC's are designated by 4000 series.

Over the years Digital logic families have evolved and thus eventually advancing the integrated circuits.
Diode Logic circuit, Resistor Transistor Logic, Diode Transistor Logic were the first logic families that were developed around 1950s.

Diode Logic(DL)

It is the most primitive of all the digital logic families. It is extremely simple and inexpensive because it only uses passive components like resistors and diodes.

The problem with the DL families is that it only provides AND & OR operation due to presence of passive components . 

Image by-Ian McKenzie 

Reason behind DL families only able to provide with AND & OR operation is absence of active elements like transistors whose basic purpose is to amplify the signal.Absence of amplification leads to absence of inversion.

Resistor-Transistor Logic (RTL)

RTL was invented around 1956.This type of device use transistor so can provide with amplification and thus the inversion process.These are not just limited to AND & OR operation,but also provide with NAND,NOR operations.

Image via-https://commons.wikimedia.org/wiki/File:RTL_NPN_NOR_Gate.svg

The circuitry consist of resistors due to which large amount of heat dissipation takes place.

Furthermore RTL has two variants that attempt to improve some of its aspects:

a]In order to save space and fabrication cost the inputs are directly connected to gate of Bipolar junction transistor.This type of RTL is known as Direct-Coupled Transistor Logic (DCTL).

b]To speed up the operation process capacitors are connected in parallel to resistors.This type of RTL is known as Resistor-Capacitor Transistor Logic (RCTL).

Diode-Transistor Logic (DTL)

DTL was invented by Orviller Baker in 1962.DTL basically helped in overcoming the power dissipation and signal degredation.The disadvantage with this circuit is the resistor attached to diodes which tends to slow down the process at which transistor is able to switch states.

Image via-https://commons.wikimedia.org/wiki/File:DTL-2NAND-simplest.svg

Like RTL,DTL also has 2 variants-
a] When a capacitor is placed in parallel with the base resistor and an inductor is placed in series with the collector resistor, DTL is known as Complemented Transistor Diode Logic (CTDL).

b]When a Zener diode and a single power supply are connected to the base of the transistor, DTL is known as High-Threshold Logic (HTL).
DTL was first used by IBM in its 1401 decimal computer in 1959.

DL, RTL and DTL have nowadays become obselete.

Emitter Coupled Logic(ECL)

ECL was invented by Hannon S. Yourke in 1956. ECL circuitry consist of resistors and transistors of type NPN.ECL are still in use because of their high performance.This high performance is achieved by minimizing the voltage drop and to prevent the transistor from entering saturation region.

Image via https://global.oup.com/us/companion.websites/fdscontent/uscompanion/us/static/companion.websites/9780199339136/pdf/Additional_Material.pdf

Motorola company was the first to introduced the first generation of ECL monolithic integrated circuits in 1962 and called it MECL I. Since then, ECL has been on the market and it’s still used today.

The main problem with ECL is that it requires high power supply for its operation and thus the heat dissipation is also high. 

Complementary Metal-Oxide Semiconductor(CMOS)

CMOS was invented by Frank Wanlass and Chih Tang Sah in 1963.CMOS is so far the most evolved logic families that is being widely used in electronics industry.This type of technology introduced a new designing approach.
CMOS considerably lower the power consumption and heat dissipation as compared to previous families. The circuit consist of both P and N type MOSFET , this enables the applied signal to turn on one of the MOSFET present and turn off the other at the same time.This momentarily switching allows less heat dissipation.

Image via https://commons.wikimedia.org/wiki/File:CMOS_inverter.svg

Microprocessors and microcontrollers are being made of CMOS devices.Because of being highly integrable circuits these are used in Very Large Scale Integrstion(VLSI).


We started with first understanding the meaning of Integrated Circuits.These IC's were themselves a family of logic circuits divided into different logic family and how the journey started from simple diode logic circuits to Complementary Metal-Oxide Semiconductor and with this evolution how at each step the circuits got more advanced.