Transistor Transistor Logic

Introduction to Digital Logic families. TTL Series, Sub-families and Comparison to Digital logic families.

1. Dayalbagh Educational Institute D.Surat M.Sc. Physics

2. Transistor Transistor Logic

3. Introduction Introduction to Digital logic gates Technologies Transistor Transistor Logic 7400 TTL Series TTL Sub-families Two Input TTL NAND Gate TTL Logic Levels and Noise Margins TTL Family Evolution Comparison to logic f amilies

4. 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.

5. 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: Resistor–transistor logic (RTL) Direct-coupled transistor logic (DCTL) Resistor–capacitor–transistor logic (RCTL)

6. Diode–transistor logic (DTL) Complemented transistor diode logic (CTDL) High-threshold logic (HTL) Emitter-coupled logic (ECL) Positive emitter-coupled logic (PECL) Low-voltage positive emitter-coupled logic (LVPECL) Transistor–transistor logic (TTL) P-type metal–oxide–semiconductor logic (PMOS) N-type metal–oxide–semiconductor logic (NMOS)

7. Complementary metal–oxide–semiconductor logic (CMOS) Bipolar complementary metal–oxide–semiconductor logic (BiCMOS) Integrated injection logic (I 2 L)

8. 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.

9. 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.

10. 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.

11. 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.

12. 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).

13. 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).

14. 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.

15. Two Input TTL NAND Gate

16. Standard TTL 74 Series Offer a combination of speed and dissipation suited for many applications. The 54 series is the counterpart of 74 series.

17. 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.

18. 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.

19. 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.

20. 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.

21. TTL Logic Levels and Noise Margins

22. 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

23. TTL family evolution

24. Comparison to logic families

25. 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.

26. 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.

27. 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).

28. 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.

29. 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.

30. Online Sources www.wikipedia.com www.allaboutcircuits.com www.electrical4u.com Books Refered FUNDAMENTALS OF DIGITAL CIRCUITS By A. ANAND KUMAR

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