Download Chapter 25

Digital Components
Chapter 25
 Introduction
 Gate Characteristics
 Logic Families
 Logic Family Characteristics
 A Comparison of Logic Families
 Complementary Metal Oxide Semiconductor
 Transistor-Transistor Logic
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
Introduction
25.1
 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
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
 Typical logic device pin-outs
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
Gate Characteristics
25.2
 The inverter or NOT gate
– consider the characteristics of a simple inverting
amplifier as shown below
– we normally use only the linear region
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
 We can use an inverting amplifier as a logical inverter
but using only the non-linear region
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
– 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)
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
 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
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
 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
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
 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
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
 The FET as a logical switch
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
 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
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
 The bipolar transistor as a logical switch
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
– 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
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
 Timing considerations
– all gates have a certain propagation delay time, tPD
– this is the average of the two switching times
tPD  21 (tPHL  tPLH )
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
Logic Families
25.3
 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)
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
Logic Family Characteristics
25.4
 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)
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
 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
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
 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
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
A Comparison of Logic Families
25.5
Parameter
CMOS
TTL
ECL
Basic gate
NAND/NOR
NAND
OR/NOR
>50
10
25
1 @ 1 MHz
1 - 22
4 - 55
Excellent
Very good
Good
1 - 200
1.5 – 33
1-4
Fan-out
Power per gate (mW)
Noise immunity
tPD (ns)
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
Complementary Metal Oxide Semiconductor
25.6
 A CMOS inverter
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
 CMOS gates
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
 CMOS logic levels and noise immunity
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
Transistor-Transistor Logic
25.7
 Discrete TTL inverter and NAND gate circuits
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
 A basic integrated circuit TTL NAND gate
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
 A standard TTL NAND gate
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
 A TTL NAND gate with open collector output
Storey: Electrical & Electronic Systems © Pearson Education Limited 2004
OHT 25.‹#›
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.‹#›