transistor-transistor logic

basketontarioΗλεκτρονική - Συσκευές

2 Νοε 2013 (πριν από 3 χρόνια και 7 μήνες)

81 εμφανίσεις

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 25.
‹#›

Digital Components


Introduction


Gate Characteristics


Logic Families


Logic Family Characteristics


A Comparison of Logic Families


Complementary Metal Oxide Semiconductor


Transistor
-
Transistor Logic



Chapter 25

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 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

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


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

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
,

V
NI

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
,
t
r

and
fall time
,
t
f


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
,
t
PD



this is the average of the two switching times

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 25.
‹#›

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

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 25.
‹#›

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)

25.4

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

Fan
-
out

>50

10

25

Power per gate (mW)

1 @ 1 MHz

1
-

22

4
-

55

Noise immunity

Excellent

Very good

Good

t
PD
(ns)

1
-

200

1.5


33

1
-

4

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 25.
‹#›

Complementary Metal Oxide Semiconductor


A CMOS inverter

25.6

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


Discrete TTL inverter and NAND gate circuits

25.7

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