Transistor Transistor Logic

Dayalbagh Educational Institute

D.Surat

M.Sc. Physics

TransistorTransistor Logic

Introduction

Introduction to Digital logic gatesTechnologies Transistor Transistor Logic7400 TTL SeriesTTL Sub-families

Two Input TTL NAND GateTTL Logic Levels and Noise Margins TTL Family EvolutionComparison to logic families

Introduction to Digital logic Gates

★ A Digital Logic Gate is an electronic device that makes logical decisions based on the different combinations of digital signals present on its inputs.

★ Digital logic gates may have more than one input but generally only have one digital output.

★ Individual logic gates can be connected together to form combinational or sequential circuits, or larger logic gate functions.

Technologies

★ The list of packaged building-block logic families can be divided into categories, listed here in roughly chronological order of introduction, along with their usual abbreviations:

o Resistor–transistor logic (RTL)• Direct-coupled transistor logic (DCTL)• Resistor–capacitor–transistor logic (RCTL)

o Diode–transistor logic (DTL)Complemented transistor diode logic (CTDL)High-threshold logic (HTL)

o Emitter-coupled logic (ECL)Positive emitter-coupled logic (PECL)Low-voltage positive emitter-coupled logic

(LVPECL)o Transistor–transistor logic (TTL)o P-type metal–oxide–semiconductor logic (PMOS)o N-type metal–oxide–semiconductor logic (NMOS)

o Complementary metal–oxide–semiconductor logic (CMOS)

o Bipolar complementary metal–oxide–semiconductor logic (BiCMOS)

o Integrated injection logic (I2L)

Transistor - Transistor Logic

★ Transistor–transistor logic (TTL) is a class of digital circuits built from bipolar junction transistors (BJT) and resistors.

★ It is called transistor–transistor logic because both the logic gating function and the amplifying function are performed by transistors.

★ The first transistor–transistor logic family of integrated circuits was introduced by Sylvania as Sylvania Universal High–Level Logic (SUHL) in 1963.

★ TTL manufacturers offered a wide range of logic gate, flip-flops, counters, and other circuits. Several variations of the original bipolar TTL concept were developed, yielding circuits with higher speed or lower power dissipation to allow optimization of a design.

★ The design of the input and outputs of TTL gates allowed many elements to be interconnected.

★ Transistor-transistor logic (TTL) is a digital logic design in which bipolar transistor s act on direct-current pulses.

★ Many TTL logic gate s are typically fabricated onto a single integrated circuit (IC).

★ TTL ICs usually have four-digit numbers beginning with 74 or 54.

★ A TTL device employs transistors with multiple emitters in gates having more than one input.

7400 TTL Series★ TTL ICs first developed in 1965 and they are known

as “standard TTL” . This version of TTL circuit is not available now.

★ TTL family use only transistor to perform the basic logic operations.

★ The standard TTL is improved to a great extent over the year.

★ TTL devices are still used as “glue” logic which connects more complex devices in digital system.

TTL Sub-families

★ There are several subfamilies or series of the TTL technology such as

★ Standard(74)★ Low-power(74L),★ High-speed(74H),★ Schottky(74S),★ Low-power Schottky(74LS),★ Advanced Schottky (74AS),★ Advanced low-power Schottky (74ALS).

Contd...

★ Commercial TTL ICs have a number designation that starts with 74 and follows with a suffix that identifies the series type. Examples are 7404, 74S86, and 74ALS161.

★ The speed-power product is an important parameter for comparing the various TTL series.

★ This is the product of the propagation delay and power dissipation and is measured in picojoules (pJ).

★ The propagation delay of a transistor circuit that goes into saturation depends mostly on two factors:○ storage time○ RC time constants.

★ A pulse through a gate take certain amount of time from input to output. This interval of time is known as propagation time.

Two Input TTL NAND Gate

Standard TTL 74 Series

★ Offer a combination of speed and dissipation suited for many applications.

★ The 54 series is the counterpart of 74 series.

Low Power TTL, 74L series

★ Essentially the same basic circuit as the standard 74 series except that all resistance values (R1=40k R2=20k R3=12k R4=500) are increased.

★ Large resistance reduce the current and therefore, the power requirement,But at the expense of reduction in speed.

★ The power consumption of Low power TTL is about 1/10 of that of standard TTL.

High Speed TTL, 74H Series

★ Essentially the same basic circuit as the standard 74 series except that smaller resistance values (R1=2.8k R2=760 R3=470 R4=58) are used and the emitter follower transistor Q3 is replaced by Darlington pair and emitter to base joining of Darlington pair (Q5-Q3) is connected to ground through a resistor of 4k.

★ The switching speed of 74H series is approximately two times more than that of standard TTL.

Schottky TTL, 74S Series ★ A major slowdown factor in BJTs is due to

transistors going in/out of saturation★ Schottky diode has a lower forward bias (0.25V)★ When BC junction would become forward biased,

the Schottky diode bypasses the current preventing the transistor from going into saturation.

Low Power Schottky TTL, 74LS Series

★ Lower power and slower speed version of 74S series.

★ It uses the schottky clamped transistor, but with larger resistance values than those in the 74S series.

TTL Logic Levels and Noise Margins

TTL Series Name

TTL Series Name Prefix

Fan-Out Power (mw) dissipation

Propagation Delay nSec

Speed Power Product pJ

Standard 74 10 10 9 90

Low-Power 74L 20 1 33 33

High-Speed 74H 10 22 6 132

Schottky 74S 10 19 3 57

Low Power Schottky

74LS 20 2 9.5 19

Advanced Schottky

74AS 40 10 1.5 15

Advanced Low Power Schottky

74ALS 20 1 4 4

TTL family evolution

Comparison to logic families

Resistor Transistor logic

★ It has a fan-out of 5.★ Propagation delay is

25ns.★ Power dissipation is

12mw.★ Noise margin for low

signal input is 0.4v.★ Poor noise immunity.★ lLow speed.

Diode Transistor Logic

★ It has fan-out of 8.★ It has high noise

immunity.★ Power dissipation is

12mw.★ Propagation constant

is average 30ns.★ Noise margin is about

0.7v.

Emitter Coupled Logic

★ Propagation delay is very low (<1ns).

★ ECL is fastest logic family.

★ ECL circuit usually operate with -ive supplies (+ive terminal is connected to ground).

Transistor Transistor Logic

★ TTl has greater speed than DTL.

★ Less noise immunity.★ Power dissipation is

10nw.★ It has fan-in of 6 and

fan-out of 10.★ Propagation time

delay is 5-15nsec.

CMOS★ Dissipates low power,

power dissipation is typically 10 nW per gate.

★ The propagation delays are usually around 25 nS to 50 nS.

★ Noise immunity approaches 50% or 45% of the full logic swing.

Online Sourceswww.wikipedia.comwww.allaboutcircuits.comwww.electrical4u.com

Books ReferedFUNDAMENTALS OF DIGITAL CIRCUITSBy A. ANAND KUMAR

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