Transistor- Transistor Logic (TTL) and their characteristics

 TTL and their characteristics

Transistor- Transistor Logic (TTL):

The Logic gates which we use are manufactured using semiconductor devices like NPN and PNP transistors, Resistors, Diodes and FETs. Each gate is integrated using different methods.

The different types of integration of logic families are Resistor- transistor Logic (RTL), Diode- Transistor Logic (DTL), Transistor- Transistor Logic (TTL), Emitter coupled Logic (EML) and Complementary metal oxide Semiconductor Logic(CMOS). RTL and DTL are not commonly used.

Transistor- Transistor Logic is made up of NPN and PNP transistor. PN junction diode and resistors are also used in the integration of the chip. TTL is widely used because it is less expensive, more reliable and faster than the other digital logic families.

Different types of Transistor- Transistor Logic are

• Standard TTL

• Low power TTL

• Schottky TTL 

• Advanced Schottky TTL

• High power TTL  

• Fast TTL

Input and Output levels of TTL: 

TTL Logic family ICs are in the series of 7400. Depending upon the integration of Logic families the IC number changes. Each method and IC has advantages and disadvantages.

Depending upon the supply voltage which is given to the IC, the voltage required to ON and OFF or to make logic ‘1’ and ‘0’ changes. For standard +5 supply voltage to obtain

• Logical ‘0’ , output voltage required is 0V-0.4V

• Logical ‘1’ , output voltage required is 2.4V-5V

• Logical ‘0’,  input voltage required is 0V-0.8V

• Logical ‘1’,  input voltage required is 2V-5V

Input and Output voltages of TTL

Internal structure and characteristics of standard TTL NAND gate:

Internal Structure of 2 input NAND Gate TTL

This is the two input TTL NAND gate circuit. It consists of four transistors Q1,Q2, Q3 and Q4. It also consists of four resistors R1,R2,R3,R4 and a diode D. 

Transistor Q1 consists of 2 emitters, two inputs are given through this 2 emitters. Q3 and Q4 transistors together form the output. To increase the number of inputs, the number of emitters at transistor Q1 is increased accordingly.

Characteristics of TTL:

Fan in and Fan out: 

Number of inputs and outputs connected to the gate, which does not affect the usual performance and does not degrade the voltage. Fan out is 10 for TTL. 

Power dissipation:

It is amount of power the device needs. It is the product of the voltage which is supplied and current needed to produce the output. It is measured in mW. Usually it is 10mW for TTL

Noise Margin:

It is the amount of noise voltage allowed at the input and it should not affect the output. The noise margin is 0.4V.

Propagation Delay:

It is the time taken from applying the input to the output produced.

Advantages:

• Power dissipation is less compared to DTL and RTL
• Less expensive
• Noise Margin and Fan out are better

Disadvantages:

• It cannot be used in high performance processors
• It is not used in high end electronic devices

Applications:

• It is used in the processors in computers
• It is used in controller circuits
• Used in remote and light controller
• Used in microprocessor and microcontrollers