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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
Thank You...
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