Chapter 4Chapter 4

Logic FamiliesLogic Families

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Chapter ObjectivesChapter Objectives

Understand the basics of digital Integrated Understand the basics of digital Integrated Circuits (IC)Circuits (IC)

Understand the different types of transistor Understand the different types of transistor logiclogic

Understand the concept of Metal-oxide Understand the concept of Metal-oxide Semiconductor (MOS) digital ICsSemiconductor (MOS) digital ICs

Understand the different types of BiCMOS Understand the different types of BiCMOS logic circuitslogic circuits

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Digital Integrated CircuitsDigital Integrated Circuits Digital ICs operate with binary signals and are Digital ICs operate with binary signals and are

constructed with ICs.constructed with ICs. Characteristics of digital ICs are:Characteristics of digital ICs are:

Speed of operationSpeed of operation Fan-inFan-in Noise immunityNoise immunity

Digital ICs can be classified into two categories:Digital ICs can be classified into two categories: MOS family: It includes the following Metal-oxide MOS family: It includes the following Metal-oxide

Semiconductor Field-effect Transistor (MOSFETs):Semiconductor Field-effect Transistor (MOSFETs): p-channel MOSFET (PMOS)p-channel MOSFET (PMOS) n-channel MOSFET (NMOS)n-channel MOSFET (NMOS) Complementary MOSFET (CMOS)Complementary MOSFET (CMOS)

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Bipolar logic families: These ICs can be classified as Bipolar logic families: These ICs can be classified as saturated and non-saturated logic families.saturated and non-saturated logic families. Saturated logic families consists of:Saturated logic families consists of:

Resistor-transistor Logic (RTL)Resistor-transistor Logic (RTL) Resistor-capacitor Transistor Logic (RCTL)Resistor-capacitor Transistor Logic (RCTL) Diode-transistor Logic (DTL)Diode-transistor Logic (DTL) High Threshold Logic (HTL)High Threshold Logic (HTL) Transistor-transistor Logic (TTL)Transistor-transistor Logic (TTL) Integrated-injection Logic (IIntegrated-injection Logic (I22L)L)

Non-saturated logic families consists of:Non-saturated logic families consists of: Schottky TTLSchottky TTL Emitter-coupled Logic (ECL)Emitter-coupled Logic (ECL)

Digital Integrated Circuits (Contd..)Digital Integrated Circuits (Contd..)

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Current-sourcing and Current-Current-sourcing and Current-sinkingsinking

The logic families are categorized on the basis of current The logic families are categorized on the basis of current flow from the output of one logic circuit to the input of flow from the output of one logic circuit to the input of another.another.

If the output of a TTL gate is HIGH, a reverse emitter If the output of a TTL gate is HIGH, a reverse emitter current of 40 mA flows from the driver gate transistor to current of 40 mA flows from the driver gate transistor to the load gate transistor. Here, the driver gate transistor is the load gate transistor. Here, the driver gate transistor is known as current source.known as current source.

If the output of the TTL gate is LOW, an emitter current 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 of 1.6 mA flows from the load gate transistor to the driver gate transistor. The driver gate transistor is known as gate transistor. The driver gate transistor is known as current sink.current sink.

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Resistor-transistor LogicResistor-transistor Logic

RTL NOR gate consists of resistors and transistors.RTL NOR gate consists of resistors and transistors. If the inputs of RTL are LOW, then the transistors are If the inputs of RTL are LOW, then the transistors are

turned OFF. Therefore, the output of the circuit is at turned OFF. Therefore, the output of the circuit is at logic 1.logic 1.

If any of the inputs are at logic 1, then the transistors are 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.turned ON. Thus, the output of the circuit is at logic 0.

Characteristics of RTL family are:Characteristics of RTL family are: Operation speed is low and average power of dissipation is Operation speed is low and average power of dissipation is

high.high. Highly sensitive to temperature but poor immunity to noise.Highly sensitive to temperature but poor immunity to noise.

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Resistor-capacitor-transistor LogicResistor-capacitor-transistor Logic

RCTL circuit consists of a capacitor with an RCTL circuit consists of a capacitor with an input resistor to increase the speed and input resistor to increase the speed and improve the immunity to noise.improve the immunity to noise.

During the transient phase, the resistor is During the transient phase, the resistor is bypassed by the capacitor. Thus, the base bypassed by the capacitor. Thus, the base current increases and the input capacitance is current increases and the input capacitance is discharged quickly.discharged quickly.

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Diode-transistor LogicDiode-transistor Logic

DTL family removes the problem of output voltage DTL family removes the problem of output voltage by increasing the load on the circuit.by increasing the load on the circuit.

If the inputs to the diodes are HIGH, they are reverse-If the inputs to the diodes are HIGH, they are reverse-biased. If both the diodes and the transistors are biased. If both the diodes and the transistors are switched ON, then the output is LOW.switched ON, then the output is LOW.

If any of the inputs to the diodes is LOW, the current If any of the inputs to the diodes is LOW, the current flows through the other diode and the voltage to that flows through the other diode and the voltage to that diode drops down. Then, the base voltage becomes diode drops down. Then, the base voltage becomes LOW and the transistor remains at logic 0 and the LOW and the transistor remains at logic 0 and the output is HIGH.output is HIGH.

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High Threshold LogicHigh Threshold Logic

HTL NAND gate is constructed by replacing HTL NAND gate is constructed by replacing the second diode from the DTL NAND gate the second diode from the DTL NAND gate with the Zener diode and increasing the power with the Zener diode and increasing the power supply.supply.

Resistor values of this gate are increased to Resistor values of this gate are increased to obtain equal amount of current from DTL and obtain equal amount of current from DTL and HTL gates.HTL gates.

These gates are useful in environments where These gates are useful in environments where the noise level is high.the noise level is high.

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Transistor-transistor LogicTransistor-transistor Logic

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

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Transistor-transistor Logic (Contd..)Transistor-transistor Logic (Contd..)

Common characteristics are:Common characteristics are: Supply voltage for all the divisions is 5.0 V.Supply voltage for all the divisions is 5.0 V. Logical 0 output voltage is 0V to 0.4V.Logical 0 output voltage is 0V to 0.4V. Logical 1 output voltage is 2.4V to 5V.Logical 1 output voltage is 2.4V to 5V. Logical 0 input voltage is 0V to 0.8V.Logical 0 input voltage is 0V to 0.8V. Logical 1 input voltage is 2V to 5V.Logical 1 input voltage is 2V to 5V. Noise immunity is 0.4 V.Noise immunity is 0.4 V.

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Integrated-injection LogicIntegrated-injection Logic

II22L is also called Merged Transistor Logic L is also called Merged Transistor Logic (MTL).(MTL).

It uses two types of bipolar junction to form a It uses two types of bipolar junction to form a large number of IC gate on the chip. Types of large number of IC gate on the chip. Types of bipolar junction are:bipolar junction are: n-p-n n-p-n p-n-p p-n-p

It dissipates less power even if it is operated at It dissipates less power even if it is operated at high speed.high speed.

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Emitter-coupled LogicEmitter-coupled Logic

ECL is a Current-mode Logic (CML).ECL is a Current-mode Logic (CML). It reduces the saturated transistor delay by It reduces the saturated transistor delay by

operating in active mode.operating in active mode. The differential amplifier, which is the basic The differential amplifier, which is the basic

circuit of ECL, draws the constant current circuit of ECL, draws the constant current during the transition of circuit from one state during the transition of circuit from one state to another.to another.

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MOS Digital Integrated CircuitsMOS Digital Integrated Circuits

MOS technology is derived from the basic MOS MOS technology is derived from the basic MOS structure of metal electrode on the oxide structure of metal electrode on the oxide insulator over a semiconductor substrate.insulator over a semiconductor substrate.

They are commonly used in digital devices such They are commonly used in digital devices such as logic gates and registers.as logic gates and registers.

They can accommodate more number of circuit They can accommodate more number of circuit elements than bipolar ICs.elements than bipolar ICs.

Operating speed of MOS IC is less than bipolar Operating speed of MOS IC is less than bipolar ICs.ICs.

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MOS Digital Integrated Circuits MOS Digital Integrated Circuits (Contd..)(Contd..)

Characteristics of MOS logic are:Characteristics of MOS logic are: Propagation delay is 50ns.Propagation delay is 50ns. Fan-out capacity of is unlimited.Fan-out capacity of is unlimited. Draw less power because of large resistance.Draw less power because of large resistance. Simplest as they use only one basic element, Simplest as they use only one basic element,

NMOS transistor.NMOS transistor.

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Complementary MOS LogicComplementary MOS Logic

They are also called COSMOS or CMOS and are made They are also called COSMOS or CMOS and are made using PMOS and NMOS transistors.using PMOS and NMOS transistors.

Characteristics of CMOS are:Characteristics of CMOS are: Power consumption is very low.Power consumption is very low. Propagation delay ranges from 25ns to 150ns that depends Propagation delay ranges from 25ns to 150ns that depends

upon the power supply.upon the power supply. CMOS can operate at voltage range of +3V to +15V.CMOS can operate at voltage range of +3V to +15V. Fan-out of CMOS is 10.Fan-out of CMOS is 10. They have small power dissipation and improved noise They have small power dissipation and improved noise

immunity.immunity.

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BiCMOS Logic CircuitsBiCMOS Logic Circuits

BiCMOS is used for developing low voltage analog BiCMOS is used for developing low voltage analog circuits, Very Large Scale Integration (VLSI) circuits circuits, Very Large Scale Integration (VLSI) circuits and Application Specific Integrated Circuits (ASIC).and Application Specific Integrated Circuits (ASIC).

Basic BiCMOS inverter circuit is formed from the Basic BiCMOS inverter circuit is formed from the complementary pairs of PMOSFET and NPMOSFET complementary pairs of PMOSFET and NPMOSFET with NPN transistors.with NPN transistors.

The switching speed of basic inverter can be The switching speed of basic inverter can be improved by discharging the excess carriers from the improved by discharging the excess carriers from the transistors with additional NMOS devices.transistors with additional NMOS devices.

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Compatibility or InterfacingCompatibility or Interfacing

The output of the circuit should match with the inputs The output of the circuit should match with the inputs of other circuits. This is referred to as compatibility.of other circuits. This is referred to as compatibility.

An interface circuit is the one, which is connected An interface circuit is the one, which is connected between the drivers and the loads.between the drivers and the loads.

The designer of the circuit should take care while The designer of the circuit should take care while matching the characteristics of current and voltage of matching the characteristics of current and voltage of two circuits of two different families.two circuits of two different families.

An interface circuit between the two families is An interface circuit between the two families is required to match the output characteristics of the required to match the output characteristics of the driver and the load.driver and the load.