TTL Logic Gates

 Digital logic circuits are manufactured depending on the specific circuit technology or
logic families.

 The different logic families are RTL (Resistor Transistor Logic), DTL(Diode Transistor Page | 1
Logic), TTL(Transistor-Transistor Logic), ECL(Emitter Coupled Logic) &
CMOS(Complementary Metal Oxide Semiconductor Logic). Out of these, RTL and DTL
are rarely used.

Transistor-Transistor Logic History

 Invented in the year 1961 by “James L. Buie of TRW”.

 Suitable for developing new integrated circuits.
 The actual name of this TTL is TCTL which means transistor-coupled transistor logic.
 In 1963, the manufacturing first commercial TTL devices were designed by “Sylvania”
known as SUHL or ‘Sylvania Universal High-Level Logic family’.
 Texas instruments launched the 5400 series ICs in the year 1964 with the range of
military temperature, then the Transistor-Transistor Logic became very popular.
 The 7400 series was launched through a narrower range in the year 1966.
 Other manufacturers are IBM, National Semiconductor, AMD, Motorola, Intel, Fairchild,
Signetics, SGS-Thomson, Siemens etc.
 Each TTL chip includes hundreds of transistors.
 Generally, functions in a single package range from logic gates to a microprocessor.

 The first PC like Kenbak-1 was used Transistor-Transistor Logic for its CPU as an
alternate of a microprocessor.
 In the year 1970, the Datapoint 2200 was used TTL components and it was the base for
the 8008 & after that the x86 instruction set.
 The GUI introduced by Xerox alto in the year 1973 as well as Star workstations in the
year 1981 were used TTL circuits which are incorporated at the level of ALUs.

What is Transistor-Transistor Logic (TTL)?

 The Transistor-Transistor Logic (TTL) is a logic family made up of BJTs (bipolar
junction transistors) and other passive components.
 A transistor performs two functions like logic as well as amplifying.
 The best examples of TTL are logic gates namely the 7402 NOR Gate & the 7400 NAND
gate.
 TTL logic includes several transistors that have several emitters as well as several inputs.

Types of Transistor-Transistor Logic

TTLs are available in different types and their classification is done based on the output like the
following:
 Standard TTL
 Fast TTL
 Schottky TTL
 High Power TTL
 Low Power TTL
 Advanced Schottky TTL.
 Low-power TTL operates with a 33ns switching speed to reduce the power consumption like 1
mW. High-speed TTL has faster switching as compared with normal TTL like 6ns. However, it
has high power dissipation like 22 mW.

Schottky TTL was launched in the year 1969 and it is used to avoid the storage of charge to
enhance the switching time by using Schottky diode clamps at the gate terminal. These gate
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terminals operate in 3ns however it includes high power dissipation like 19 mW

Low power TTL uses high resistance values from low power TTL. The Schottky diodes will
provide a good blend of speed as well as decreased power utilization like 2 mW. This is the most
general type of TTL, used like glue logic within microcomputers, basically replaces the past sub-
families like L, H & S.

The fast TTL is used to increase the transition from low-to-high. These families attained PDPs of
4pJ & 10 pJ, correspondingly. LVTTL or Low-voltage TTL for 3.3V power supplies as well as
memory interfacing.

Most of the designers provide commercial as well as extensive temperature ranges. For instance,
the temperature range of 7400 series parts from Texas Instruments ranges from 0 – 70 °C as well
as 5400 series temperature range is from −55 to +125 °C. The parts with high reliability and
special quality are accessible for aerospace & military applications whereas the radiation devices
from the SNJ54 series are used in space applications.

Characteristics of TTL
The characteristics of TTL include the following.

1. Fan in: Number of inputs that can practically be supported without degrading practically input
voltage level.
2. Fan Out: Number of loads the output of a GATE can drive without affecting its usual
performance. By load we mean the amount of current required by the input of another Gate
connected to the output of the given gate.
3. Power Dissipation: It represents the amount of power needed by the device. It is measured in
mW. It is usually the product of supply voltage and the amount of average current drawn when
the output is high or low.
4. Propagation Delay: It represents the transition time that elapses when the input level changes.
The delay which occurs for the output to make its transition is the propagation delay.
5. Noise Margin: It represents the amount of noise voltage allowed at the input, which doesn’t
affect the standard output.

Basic Features of Interest

VOH(min) – The minimum voltage level at an output in the logical “1” state under
defined load conditions
VOL(max) – The maximum voltage level at an output in the logical “0” state under
defined load conditions
VIH(min) – The minimum voltage required at an input to be recognized as “1” logical
state
VIL(max) – The maximum voltage required at an input that still will be recognized as
“0” logical state
 IOH – Current flowing into an output in the logical “1” state under specified load
conditions
IOL – Current flowing into an output in the logical “0” state under specified load
conditions
IIH – Current flowing into an input when a specified HI level iiss applied to that input
IIL – Current flowing into an input when a specified LO level is applied to that input Page | 3

Classification of Transistor
Transistor-Transistor Logic
A. Open Collector Output
The main feature is that its output is 0 when low and floating when high. Usually, an external Vcc
may be applied.

Open Collector Output of Transistor

Transistor-Transistor Logic

Transistor Q1 behaves as a cluster of diodes placed back to back. With any of the input at logic
low, the corresponding emitter-basebase junction is forward biased and the voltage drop across the
base of Q1 is around 0.9V, not enough for the transistors Q2 and Q3 to conduct. Thus the output
is either floating or Vcc, i.e. High level.

Similarly, when all inputs are high, all basebase-emitter

emitter junctions of Q1 are reverse biased
b and
transistor Q2 and Q3 get enough base current and are in saturation mode. The output is at logic
low. (For a transistor to go to saturation, collector current should be greater than β times the base
current).

Applications
The applications of open collector output include the following.

 In driving lamps or relays

 In performing wired logic
 In the construction of a common bus system
 B. Totem Pole Output
Totem Pole means the addition of an active pull up the circuit in the output of the Gate which
results in a reduction of propagation delay.

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Totem Pole Output TTL

Logic operation is the same as the open collector output. The use of transistors Q4 and diode is to
provide quick charging and discharging of parasitic capacitance across Q3. The resistor is used to
keep the output current to a safe value.

C. Three State Gate

It provides 3 state output like the following

 Low-level
level state when a lower transistor is ON and an upper transistor is OFF.
 High-level
level state when the lower transistor is OFF and the upper transistor is ON.
 Third state when both transistors are OFF. It allows a direct wire connection of many outputs.

Three State Gate Transistor-Transistor

Transistor Logic

TTL Family Features

The features of the TTL family include the following.

 Logic low level is at 0 or 0.2V.

 Logic high level is at 5V.
 Typical fan out of 10. It means it can support at most 10 gates at its output.
 A basic TTL device draws a power of almost 10mW, which reduces with the use of Schottky
devices.
 The average propagation delay is about 9ns.
 The noise margin is about 0.4V.

Typical TTL Circuits

The 3 basic Logic gates implemented using TTL logic are given below:
below:- Page | 5

NOR Gate

Suppose input A is at logic high, the corresponding transistor’s emitter-base

base junction is reverse
biased, and base-collector
collector junction is forward biased. Transistor Q3 gets base current from supply
voltage Vcc and goes to saturation. As a result of the low collector voltage from Q3, transistor
transis Q5
goes to cut off and on the other hand, if another input is low, Q4 is cut off and correspondingly
Q5 is cut off and output is connected directly to the ground through transistor Q3. Similarly,
when both inputs are logic low, the output will be at llogic high.

TTL - NOR Gate

NOT Gate

When the input is low, the corresponding base-emitter

base emitter junction is forward biased, and the base
base-
collector junction is reverse biased. As a result transistor Q2 is cut off and also transistor Q4 is
cut off. Transistor Q3 goes to saturation and diode D2 starts conducting and output is connected
to Vcc and goes to logic high. Similarly, when input is at logic high, the output is at logic low.
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TTL - NOT Gate

Comparison of TTL with Other Logic Families

 Use
se more power as compared with CMOS devices, but power utilization does not
enhance through clock speed for CMOS devices.
 TTL is low-sensitive
sensitive to damage from electrostatic discharge as compared to early CMOS
devices.
 Because of the TTL device’s o/p struct
structure,
ure, the o/p impedance is asymmetrical among the
low and high states to make them inappropriate to drive transmission lines. Usually, this
drawback overcomes through buffering the o/p using special line-driver
line driver devices wherever
signals require transmitting throughout cables.
 TTL systems use a decoupling capacitor for each one otherwise two IC packages, so a
current signal from one TTL chip doesn’t decrease the voltage supply voltage to another
momentarily.

The comparison of TTL with other logic families’:

families’

Specifications TTL CMOS ECL

NAND OR/NOR
Basic Gate NOR/NAND
Passive Elements & Passive Elements &
Components Transistors MOSFETs Transistors

10 25
Fan-out >50
 Strong Good
Noise Immunity Extremely Strong

Moderate Low
Noise Margin High
1.5 to 30 1 to 4
TPD in ns 1 to 210 Page | 7
35 >60
Clock Rate in MHz 10
10 40 to 55
Power/Gate in mWatt 0.0025
100 40 to 50
Figure of Merit 0.7

Advantages and Disadvantages

 The advantages of disadvantages of TTL include the following.
 We can easily interface with other circuits & the ability to generate difficult logic
functions because of certain voltage levels as well as good noise margins
 Good Fan in
 Immune to harm from stationary electricity discharges not like CMOS & as compared to
CMOS these are economical.
 The main drawback of TTL is high current utilization.
 With the arrival of CMOS, TTL applications have been replaced through CMOS. But,
TTL is still utilized in applications because they are quite robust & the logic gates are
fairly cheap.

TTL Applications
The applications of TTL include the following.

 Used in controller application for providing 0 to 5Vs

 Used as a switching device in driving lamps and relays
 Used in processors of mini computers like DEC VAX
 Used in printers and video display terminals