BRANCH-E&TC ENGINEERING

SEM – 3Rd

SUBJECT-DIGITAL ELECTRONICS

CHAPTER-06- logic family

TOPIC- ttl

Ay-2021-2022 ,winter-2021

FACULTY-ER S MOHANTA.(Hod e & tc engg Dept)

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Presentation On:-

TTL

Digital Components

• Introduction
• Gate Characteristics
• Logic Families
• Logic Family Characteristics
• A Comparison of Logic Families
• Complementary Metal Oxide Semiconductor
• Transistor-Transistor Logic

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Introduction

• Earlier we looked at a range of digital applications based on logic gates – at that time we treated the gates as ‘black boxes’
• We will now consider the construction of such gates, and their characteristics
• In this lecture we will concentrate on small- and medium-scale integration circuits containing just a handful of gates
• typical gates are shown on the next slide

• Typical logic device pin-outs

Gate Characteristics

• The inverter or NOT gate
• consider the characteristics of a simple inverting amplifier as shown below
• we normally use only the linear region

• We can use an inverting amplifier as a logical inverter but using only the non-linear region

• we choose input values to ensure that we are always outside of the linear region – as in (a)
• unlike linear amplifiers, we use circuits with a rapid transition between the non-linear regions – as in (b)

• Logic levels
• the voltage ranges representing ‘0’ and ‘1’ represent the logic levels of the circuit
• often logic 0 is represented by a voltage close to 0 V but the allowable voltage range varies considerably
• the voltage used to represent logic 1 also varies greatly. In some circuits it might be 2-4 V, while in others it might be 12-15 V
• in order for one gate to work with another the logic levels must be compatible

• Noise immunity
• noise is present in all real systems
• this adds random fluctuations to voltages representing logic levels
• to cope with noise, the voltage ranges defining the logic levels are more tightly constrained at the output of a gate than at the input
• thus small amounts of noise will not affect the circuit
• the maximum noise voltage that can be tolerated by a circuit is termed its noise immunity, V_NI

• Transistors as switches
• both FETs and bipolar transistors make good switches
• neither form produce ideal switches and their characteristics are slightly different
• both forms of device take a finite time to switch and this produces a slight delay in the operation of the gate
• this is termed the propagation delay of the circuit

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• The FET as a logical switch

• Rise and fall times
• because the waveforms are not perfectly square we need a way of measuring switching times
• we measure the rise time, t_r and fall time, t_f as shown below

• The bipolar transistor as a logical switch

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• when the input voltage to a bipolar transistor is high the transistor turns ON and the output voltage is driven down to its saturation voltage which is about 0.1 V
• however, saturation of the transistor results in the storage of excess charge in the base region
• this increases the time taken to turn OFF the device – an effect known as storage time
• this makes the device faster to turn ON than OFF
• some switching circuits increase speed by preventing the transistors from entering saturation

• Timing considerations
• all gates have a certain propagation delay time, t_PD
• this is the average of the two switching times

Logic Families

• We have seen that different devices use different voltages ranges for their logic levels
• They also differ in other characteristics
• In order to assure correct operation when gates are interconnected they are normally produced in families
• The most widely used families are:
• complementary metal oxide semiconductor (CMOS)
• transistor-transistor logic (TTL)
• emitter-coupled logic (ECL)

Logic Family Characteristics

• Complementary metal oxide semiconductor (CMOS)
• most widely used family for large-scale devices
• combines high speed with low power consumption
• usually operates from a single supply of 5 – 15 V
• excellent noise immunity of about 30% of supply voltage
• can be connected to a large number of gates (about 50)
• many forms – some with t_PD down to 1 ns
• power consumption depends on speed (perhaps 1 mW)

• Transistor-transistor logic (TTL)
• based on bipolar transistors
• one of the most widely used families for small- and medium-scale devices – rarely used for VLSI
• typically operated from 5V supply
• typical noise immunity about 1 – 1.6 V
• many forms, some optimised for speed, power, etc.
• high speed versions comparable to CMOS (~ 1.5 ns)
• low-power versions down to about 1 mW/gate

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• Emitter-coupled logic (ECL)
• based on bipolar transistors, but removes problems of storage time by preventing the transistors from saturating
• very fast operation - propagation delays of 1ns or less
• high power consumption, perhaps 60 mW/gate
• low noise immunity of about 0.2-0.25 V
• used in some high speed specialist applications, but now largely replaced by high speed CMOS

A Comparison of Logic Families

Complementary Metal Oxide Semiconductor

• A CMOS inverter

• CMOS gates

• CMOS logic levels and noise immunity

Transistor-Transistor Logic

• Discrete TTL inverter and NAND gate circuits

• A basic integrated circuit TTL NAND gate

• A standard TTL NAND gate

• A TTL NAND gate with open collector output