Logic Gates

Logic Families

INTRODUCTION

ICs are integrated using following integration techniques•SSI < 12 gates/chip•MSI < 100 gates/chip•LSI …1K gates/chip•VLSI …10K gates/chip•ULSI …100K gates/chip

Logic Family

It is a group of compatible ICs with the same logic levels and the supply voltages for performing various logic functions

They are the building block of logic circuits.

Types of logic families

BIPOLAR ICs

The main element of a bipolar ICs are Resistors, Diodes, Capacitors and Transistors.

They can be operated in two ways:SaturatedNon-Saturated

Saturated Logic: The transistors in the IC are driven to saturation

Non-Saturated Logic: The transistors in the IC are not driven to saturation.

BIPOLAR ICsSaturated Bipolar families are:

Resistor-Transistor logic (RTL),Direct-Coupled transistor logic (DCTL),Integrated-injection logic (IIL),Diode-transistor logic (DTL),High-Threshold logic (HTL) andTransistor-transistor logic (TTL)

Non-Saturated Bipolar families are:Schottkey TTLEmitter-Coupled logic (ECL)

UNIPOLAR LOGIC FAMILIES

MOS devices are unipolar devices and only MOSFETs are employed in MOS logic circuits.

These families are:PMOS (p-channel MOSFETs)NMOS (n-channel MOSFETs)CMOS (Both p- and n- channel MOSFETs are

fabricated on same silicon chip)

Basic Characteristics of ICsPropagation delayPower dissipationFan in and fan outNoise immunityPower supply requirementFigure of merits i.e. speed power productOperating temperatureCurrent and voltage parameters

Power supply requirement

CMOS and TTL are available in different supply voltage categories

In each IC, Vcc pin is connected to positive supply and GND pin is connected to ground of supply.

Current and voltage parameters

Four different kind of Logic level specifications are defined: VIL, VIH, VOL, VOH

VIL, VIH : These are the input logic levels (Low & High)

VOL, VOH : These are the output logic levels (Low & High)

Noise Immunity

Noise is unwanted voltage that is induced in electrical circuits and can cause threat to proper operation of circuit.

Noise immunity is the ability to tolerate a certain amount of unwanted voltage fluctuations on its inputs without changing outputs

Noise MarginA quantitative measure of noise immunity is

called Noise margin. It is expressed in volts.Two Noise margins are specified for logic circuits,

High level Noise margin (VNH) and Low level Noise margin (VNL), expressed as:

Power Dissipation

This is the amount of power dissipated in an IC.

It is Determined by the current Icc, that it draws from the Vcc supply, and is given by , Pd = Vcc X Icc.

Propagation Delay

Fan in and Fan outFan-in:

Number of inputs a gate has. For example, a two input gate will have fan-in equal to 2

Fan-out:

Maximum number of inputs of the same IC family that a gate can drive without falling outside the specified output voltage limits.

Figure of merits/speed power product

This is obtained by multiplying gate propagation delay by gate power dissipation

Operating temperatureFor consumer application , the temperature is

0 to +70°CFor industrial application , the temperature is

-55°C to +125 °C

Transistor as a switch

Transistor in cut off

Transistor as a switch

Transistor in saturation

TTL(Transistor Transistor logic)

Diode equivalent of TTL with single emitter transistor

Diode equivalent of a TTL multiple emitter Transistor

TTL Inverter

Basic integrated circuit TTL NAND gate

Basic TTLNANDNAND gate.

CMOS-Complementary metal oxide semiconductor

most widely used family for large-scale devices

combines high speed with low power consumption

usually operates from a single supply of 5 – 15 V

excellent noise immunity of about 30% of supply voltage

CMOS inverterA Q1 Q2 O/P

0V(Logic 0)

OFF ON VDD

(Logic 1)

VDD

(Logic 1) ON OFF 0V

(Logic 0)

CMOS NAND gateInput Transistor O/

P

A B Q1 Q2 Q3 Q4 Y

0 0 ON ON OFF OFF 1

0 1 ON OFF OFF ON 1

1 0 OFF ON ON OFF 1

1 1 OFF OFF ON ON 0

CMOS AND gate

CMOS NOR gate

CMOS OR gate