What is Transistor Transistor Logic (TTL) & Its Working

Logic Gates like NAND, NOR are used in daily applications for performing logic operations. The Gates are manufactured using semiconductor devices like BJT, Diodes, or FETs. Different Gate’s are constructed using Integrated circuits. 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 Logic), TTL(Transistor-Transistor Logic), ECL(Emitter Coupled Logic) & CMOS(Complementary Metal Oxide Semiconductor Logic). Out of these, RTL and DTL are rarely used. This article discusses an overview of a Transistor-Transistor Logic or TTL.

Transistor-Transistor Logic History

The TTL or Transistor-Transistor Logic logic was invented in the year 1961 by “James L. Buie of TRW”. It is 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’.

After the Texas instruments engineers launched the 5400 series ICs in the year 1964 with the range of military temperature, then the Transistor-Transistor Logic became very popular. After that, the 7400 series was launched through a narrower range in the year 1966.

The compatible parts of the 7400 families launched by Texas instruments were designed by several companies like National Semiconductor, AMD, Motorola, Intel, Fairchild, Signetics, Intersil, Mullard, SGS-Thomson, Siemens, Rifa, etc. The one and only one manufacturing company like IBM was launched non-compatible circuits using TTL for their own use.

The Transistor-Transistor Logic was applied to many bipolar logic generations by slowly improving the speed as well as power utilization over about two decades. Usually, 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). As the name suggests, the 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. The types of TTL or transistor-transistor logic mainly include Standard TTL, Fast TTL, Schottky TTL, High power TTL, Low power TTL & Advanced Schottky TTL.

The designing of TTL logic gates can be done with resistors and BJTs. There are several variants of TTL which are developed for different purposes such as the radiation-hardened TTL packages for space applications and Low power Schottky diodes that can provide an excellent combination of speed and lesser power consumption.

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. At present, this was replaced through CMOS logic. 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 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 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.
2. 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.
3. 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.
4. Noise Margin: It represents the amount of noise voltage allowed at the input, which doesn’t affect the standard output.
   

Classification of Transistor-Transistor Logic

It is a logical family consisting completely of transistors.  It employs a transistor with multiple emitters.  Commercially it starts with the 74 series like the 7404, 74S86, etc.  It was built in 1961 by James L Bui and commercially used in logic design in 1963. TTLs are classified based on the output.

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.

Transistor Q1 behaves as a cluster of diodes placed back to back. With any of the input at logic low, the corresponding emitter-base 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 base-emitter junctions of Q1 are reverse biased 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

 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.

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.

Three State Gate

It provides 3 state output like the following

• Low-level state when a lower transistor is ON and an upper transistor is OFF.
• High-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.

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.

Series of TTL IC

TTL ICs mostly start with the 7 series.  It has 6 subfamilies given as:

1. Low Power device with a propagation delay of 35 ns and power dissipation of 1mW.
2. Low power Schottky device with a delay of 9ns
3. Advanced Schottky device with a delay of 1.5ns.
4. Advanced low power Schottky device with a delay of 4 ns and power dissipation of 1mW.

In any TTL device nomenclature, the first two names indicate the name of the subfamily the device belongs to. The first two digits indicate the temperature range of operation. The next two alphabets indicate the subfamily the device belongs to. The last two digits indicate the logic function performed by the chip. The examples are 74LS02- 2 neither input NOR gate, 74LS10- Triple 3 input NAND gate.

Typical TTL Circuits

Logic Gates are used in daily life in applications like a clothes dryer, computer printer, doorbell, etc.

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

 NOR Gate

Suppose input A is at logic high, the corresponding transistor’s emitter-base junction is reverse biased, and base-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 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 logic high.

NOT Gate

When the input is low, the corresponding base-emitter junction is forward biased, and the 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.

TTL Comparison with Other Logic Families

Generally, TTL devices use more power as compared with CMOS devices, but power utilization does not enhance through clock speed for CMOS devices. As compared to current ECL circuits, transistor-transistor logic uses low power but has simple design rules but it is significantly slower.

Manufacturers can unite TTL & ECL devices within the same system to attain the best performance, but devices like level-shifting are necessary among the two logic families. TTL is low-sensitive to damage from electrostatic discharge as compared to early CMOS devices.

Because of the TTL device’s o/p structure, 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 devices wherever signals require transmitting throughout cables.

The totem-pole o/p structure of TTL frequently has a quick overlap once both the higher & lower transistors are conducting which results in a substantial signal of current drawn from the power supply.

These signals can connect in sudden methods among several IC packages, which results in lower performance & reduced noise margin. Generally, the 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.

At present, many designers supplying CMOS logic equivalents through TTL compatible i/p & o/p levels through part numbers that are related to the corresponding TTL component including the same pinouts. So for instance, the 74HCT00 series will provide several drop-in alternates for 7400 bipolar series parts, however utilizes CMOS technology.

The comparison of TTL with other logic families’ in terms of different specifications includes the following.

Specifications	TTL	CMOS	ECL
Basic Gate	NAND	NOR/NAND	OR/NOR
Components	Passive Elements & Transistors	MOSFETs	Passive Elements & Transistors
Fan-out	10	>50	25
Noise Immunity	Strong	Extremely Strong	Good
Noise Margin	Moderate	High	Low
TPD in ns	1.5 to 30	1 to 210	1 to 4
Clock Rate in MHz	35	10	>60
Power/Gate in mWatt	10	0.0025	40 to 55
Figure of Merit	100	0.7	40 to 50

Transistor-Transistor Logic Inverter

The transistor Transistor Logic (TTL) devices have replaced diode transistor logic (DTL) as they work quicker & are cheaper to function. The NAND IC with Quad 2-input uses a 7400 TTL device to design a wide range of circuits which is used as an inverter.

The circuit diagram above uses NAND gates within the IC. So select switch A to activate the circuit then you can notice that both the LEDs in the circuit will turn off. When the output is low, then the input should be high. After that, select the switch B then both the LEDs will turn ON.

When switch A has selected then both the inputs of the NAND gate will be high, which means the output of the logic gates will be less. When switch B is selected then the inputs will not be high for a long time & the LEDs will turn ON.

Advantages and Disadvantages

The advantages of disadvantages of TTL include the following.

The main benefit of TTL is 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 TTL has good features like fan-in which means the number of i/p signals that can be accepted through an input.

TTL is mainly 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. The TTL’s high current demands can lead to offensive functioning because o/p states will be turned off. Even with different TTL versions that have low current consumption will be competitive to CMOS.

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

Thus, this is all about an overview of TTL or Transistor-Transistor logic. It is a group of ICs that keep logic states as well as to attain switching using BJTs. TTL is one of the reasons that ICs are so extensively used because they are inexpensive, faster, and high reliability as compared to TTL and DTL. A TTL uses transistors through several emitters in gates that have several inputs. Here, is a question for you, what are the sub-categories of transistor-transistor logic?