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Chapter 4

Chapter 4. Logic Families. Chapter Objectives. Understand the basics of digital Integrated Circuits (IC) Understand the different types of transistor logic Understand the concept of Metal-oxide Semiconductor (MOS) digital ICs Understand the different types of BiCMOS logic circuits.

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Chapter 4

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  1. Chapter 4 Logic Families

  2. Chapter Objectives • Understand the basics of digital Integrated Circuits (IC) • Understand the different types of transistor logic • Understand the concept of Metal-oxide Semiconductor (MOS) digital ICs • Understand the different types of BiCMOS logic circuits

  3. Digital Integrated Circuits • Digital ICs operate with binary signals and are constructed with ICs. • Characteristics of digital ICs are: • Speed of operation • Fan-in • Noise immunity • Digital ICs can be classified into two categories: • MOS family: It includes the following Metal-oxide Semiconductor Field-effect Transistor (MOSFETs): • p-channel MOSFET (PMOS) • n-channel MOSFET (NMOS) • Complementary MOSFET (CMOS)

  4. Digital Integrated Circuits (Contd..) • Bipolar logic families: These ICs can be classified as saturated and non-saturated logic families. • Saturated logic families consists of: • Resistor-transistor Logic (RTL) • Resistor-capacitor Transistor Logic (RCTL) • Diode-transistor Logic (DTL) • High Threshold Logic (HTL) • Transistor-transistor Logic (TTL) • Integrated-injection Logic (I2L) • Non-saturated logic families consists of: • Schottky TTL • Emitter-coupled Logic (ECL)

  5. Current-sourcing and Current-sinking • The logic families are categorized on the basis of current flow from the output of one logic circuit to the input of another. • If the output of a TTL gate is HIGH, a reverse emitter current of 40 mA flows from the driver gate transistor to the load gate transistor. Here, the driver gate transistor is known as current source. • If the output of the TTL gate is LOW, an emitter current of 1.6 mA flows from the load gate transistor to the driver gate transistor. The driver gate transistor is known as current sink.

  6. Resistor-transistor Logic • RTL NOR gate consists of resistors and transistors. • If the inputs of RTL are LOW, then the transistors are turned OFF. Therefore, the output of the circuit is at logic 1. • If any of the inputs are at logic 1, then the transistors are turned ON. Thus, the output of the circuit is at logic 0. • Characteristics of RTL family are: • Operation speed is low and average power of dissipation is high. • Highly sensitive to temperature but poor immunity to noise.

  7. Resistor-capacitor-transistor Logic • RCTL circuit consists of a capacitor with an input resistor to increase the speed and improve the immunity to noise. • During the transient phase, the resistor is bypassed by the capacitor. Thus, the base current increases and the input capacitance is discharged quickly.

  8. Diode-transistor Logic • DTL family removes the problem of output voltage by increasing the load on the circuit. • If the inputs to the diodes are HIGH, they are reverse-biased. If both the diodes and the transistors are switched ON, then the output is LOW. • If any of the inputs to the diodes is LOW, the current flows through the other diode and the voltage to that diode drops down. Then, the base voltage becomes LOW and the transistor remains at logic 0 and the output is HIGH.

  9. High Threshold Logic • HTL NAND gate is constructed by replacing the second diode from the DTL NAND gate with the Zener diode and increasing the power supply. • Resistor values of this gate are increased to obtain equal amount of current from DTL and HTL gates. • These gates are useful in environments where the noise level is high.

  10. Transistor-transistor Logic • The 54/74 series of TTL family is classified into 5 divisions: • Standard (SN 54/74) • High-speed (SN54H/74H) • Low-power (SN54L/74L) • Schottky-diode-clamped (SN54S/74S) • Low power schottky (SN54LS/74LS)

  11. Transistor-transistor Logic (Contd..) • Common characteristics are: • Supply voltage for all the divisions is 5.0 V. • Logical 0 output voltage is 0V to 0.4V. • Logical 1 output voltage is 2.4V to 5V. • Logical 0 input voltage is 0V to 0.8V. • Logical 1 input voltage is 2V to 5V. • Noise immunity is 0.4 V.

  12. Integrated-injection Logic • I2L is also called Merged Transistor Logic (MTL). • It uses two types of bipolar junction to form a large number of IC gate on the chip. Types of bipolar junction are: • n-p-n • p-n-p • It dissipates less power even if it is operated at high speed.

  13. Emitter-coupled Logic • ECL is a Current-mode Logic (CML). • It reduces the saturated transistor delay by operating in active mode. • The differential amplifier, which is the basic circuit of ECL, draws the constant current during the transition of circuit from one state to another.

  14. MOS Digital Integrated Circuits • MOS technology is derived from the basic MOS structure of metal electrode on the oxide insulator over a semiconductor substrate. • They are commonly used in digital devices such as logic gates and registers. • They can accommodate more number of circuit elements than bipolar ICs. • Operating speed of MOS IC is less than bipolar ICs.

  15. MOS Digital Integrated Circuits (Contd..) • Characteristics of MOS logic are: • Propagation delay is 50ns. • Fan-out capacity of is unlimited. • Draw less power because of large resistance. • Simplest as they use only one basic element, NMOS transistor.

  16. Complementary MOS Logic • They are also called COSMOS or CMOS and are made using PMOS and NMOS transistors. • Characteristics of CMOS are: • Power consumption is very low. • Propagation delay ranges from 25ns to 150ns that depends upon the power supply. • CMOS can operate at voltage range of +3V to +15V. • Fan-out of CMOS is 10. • They have small power dissipation and improved noise immunity.

  17. BiCMOS Logic Circuits • BiCMOS is used for developing low voltage analog circuits, Very Large Scale Integration (VLSI) circuits and Application Specific Integrated Circuits (ASIC). • Basic BiCMOS inverter circuit is formed from the complementary pairs of PMOSFET and NPMOSFET with NPN transistors. • The switching speed of basic inverter can be improved by discharging the excess carriers from the transistors with additional NMOS devices.

  18. Compatibility or Interfacing • The output of the circuit should match with the inputs of other circuits. This is referred to as compatibility. • An interface circuit is the one, which is connected between the drivers and the loads. • The designer of the circuit should take care while matching the characteristics of current and voltage of two circuits of two different families. • An interface circuit between the two families is required to match the output characteristics of the driver and the load.

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