# transistor-transistor logic

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Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

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Digital Components

Introduction

Gate Characteristics

Logic Families

Logic Family Characteristics

A Comparison of Logic Families

Complementary Metal Oxide Semiconductor

Transistor
-
Transistor Logic

Chapter 25

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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

25.1

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Typical logic device pin
-
outs

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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

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

region

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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)

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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

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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

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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

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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

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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

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Timing considerations

all gates have a certain
propagation delay time
,
t
PD

this is the average of the two switching times

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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

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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

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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

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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

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Complementary Metal Oxide Semiconductor

A CMOS inverter

25.6

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CMOS gates

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CMOS logic levels and noise immunity

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Transistor
-
Transistor Logic

Discrete TTL inverter and NAND gate circuits

25.7

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A basic integrated circuit TTL NAND gate

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A standard TTL NAND gate

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A TTL NAND gate with open collector output

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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