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Digital System�Lecture - 09

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LOGIC FAMILIES

  • Most electronic systems which are responsible for modern advances are based on digital technology. All digital systems, computers and microprocessors are assembled from simple circuits called logic circuits. The basic building blocks of logic circuits are logic gates. And logic gates themselves are simple electronic circuits comprising of diodes, transistors and resistors.

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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.

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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.

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  • 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

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Input and Output levels of TTL:

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

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

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

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

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Internal structure and characteristics of standard TTL NAND gate:

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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.

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Transistor - Transistor Logic (TTL)

  • 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

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Emitter Coupled Logic (ECL)

  • Emitter Coupled Logic (ECL) is known as non-saturated logic.

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  • It has propagation delay time varying from 0.75ns to 2ns.

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  • However, power dissipation is increased since one transistor is always in the active region.

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Circuit Operation of Emitter Coupled Logic

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Features of Emitter Coupled Logic

  • It provides two outputs which are always complements of each other (figure 1).

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  • It is so because the circuit operation is based on a differential amplifier.

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  • This family is particularly suited to monolithic fabrication techniques because logic levels are function of resistor ratios.

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Typical characteristic of an ECL family

  • Propagation delay time per gate of 1 ns (meaning extremely fast speed),

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  • Power dissipation of 30 mW,

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  • Fan-out of 50,

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  • Fan-in of 5,

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  • Noise immunity 0.4V.

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Emitter Coupled Logic (ECL)

  • Advantages of CMOS:
    • More speedy
    • Small voltage swing
    • Fastest switching
    • Constant current supply
    • Noise immunity
  • Disadvantages of CMOS:
    • Low fan-out
    • Input-Output voltage variation not matching

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Complementary Metal Oxide Semiconductor Logic (CMOS)

  • CMOS stands for “Complementary metal Oxide Semiconductor”. It is also known as “complimentary-symmetry metal Oxide Semiconductor COS-MOS”.

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  • It is widely used in the integration of chips (ICs). It is used in Computer memories like RAM, ROM, EEPROM, cell phones, microprocessors and microcontrollers.

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Structure of CMOS

  • The power dissipation and consumption is very less in CMOS and it is faster, so it is widely used than the bipolar circuits. CMOS consists of P- channel MOSFET (PMOS) and N-channel MOSFET (NMOS).

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Structure of CMOS

  • Both NMOS and PMOS together design the logic function. Same input voltage is used to turn ON one MOSFET and turn OFF other MOSFET. So there is no need of pull up resistor in CMOS.

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  • NMOS is arranged in the pull down network between the output and the ground. PMOS is arranged in the pull up network. This pull up and pull down network is arranged in such a way that when one network is ON, the other network will be OFF.

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Characteristics of CMOS

  • 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 in and Fan out is usually 10 for CMOS.
  • 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 at 1MHZ and 0.1mW at 100KHZ.
  • Noise Margin: It is the amount of noise voltage allowed at the input and it should not affect the output. The noise margin in CMOS is 45% of the supply voltage. It is usually 2.25V for 5V input.
  • Propagation Delay: It is the time taken from applying the input to the output produced. It is normally 25 to 150ns.

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Complementary Metal Oxide Semiconductor Logic (CMOS)

  • Advantages of CMOS:
    • Power consumption is less
    • Large fan-out capability
    • High noise immunity and noise margin
    • Power dissipation is low
    • Faster than NMOS
  • Disadvantages of CMOS:
    • Manufacturing cost is high
    • Propagation delay is higher than TTL and ECL

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Complementary Metal Oxide Semiconductor Logic (CMOS)

  • Applications of CMOS:
    • Analog to digital converter
    • Image sensors
    • Amplifiers
    • Static RAM
    • Registers
    • Microchip
    • Microprocessors and microcontrollers
    • Transceivers

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Thanks!

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