storey: electrical & electronic systems © pearson education limited 2004 oht 25.1 digital...

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 25.1

Digital Components

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

Chapter 25


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

25.1


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

25.2


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, VNI


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


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, tr and fall time, tf as

shown below


The bipolar transistor as a logical switch


– 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, tPD

– this is the average of the two switching times

)t(tt PLHPHLPD 21


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)

25.3


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 tPD down to 1 ns

– power consumption depends on speed (perhaps 1 mW)

25.4


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


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 25.5

Parameter CMOS TTL ECL

Basic gate NAND/NOR NAND OR/NOR

Fan-out >50 10 25

Power per gate (mW) 1 @ 1 MHz 1 - 22 4 - 55

Noise immunity Excellent Very good Good

tPD (ns) 1 - 200 1.5 – 33 1 - 4


Complementary Metal Oxide Semiconductor

A CMOS inverter

25.6


CMOS gates


CMOS logic levels and noise immunity


Transistor-Transistor Logic

Discrete TTL inverter and NAND gate circuits

25.7


A basic integrated circuit TTL NAND gate


A standard TTL NAND gate


A TTL NAND gate with open collector output


Key Points

Physical gates are not ideal components Logic gates are manufactured in a range of logic families The ability of a gate to ignore noise is its ‘noise immunity’ Both MOSFETs and bipolar transistors are used in gates All logic gates exhibit a propagation delay when

responding to changes in their inputs The most widely used logic families are CMOS and TTL CMOS is available in a range of forms offering high speed

or very low power consumption TTL logic is also produced in many versions, each

optimised for a particular characteristic

storey: electrical & electronic systems © pearson education limited 2004 oht 25.1 digital...

Documents

logic gates

logic levels

circuit slide

saturation slide

logic ecl

real systems

b slide

compatible slide