LOGIC GATE

                                 LOGIC GATE

 LOGIC GATE

LOGIC GATES AND MEMORY DEVICES ARE FABRICATED AS INTEGRATED CIRCUITS BECAUSE THE COMPONENTS USED IN, SUCH AS RESISTORS, DIODE, BIPOLAR JUNCTION TRANSISTORS, AND THE INSULATED GATE OR MOSFET ARE THE INTEGRAL PARTS OF THE CHIP.

DIGITAL ICs ARE OFTEN CATEGORIZED ACCORDING TO THEIR CIRCUIT COMPLEXITY AS MEASURED BY THE NUMBER OF LOGIC GATES IN A SINGLE PACKAGE. INTEGRATED CIRCUITS ARE CLASSIFIED INTO TWO GENERAL CATEGORIES:-

1. LINEAR INTEGRATED CIRCUITS

2. DIGITAL INTEGRATED CIRCUITS

LINEAR INTEGRATED CIRCUITS OPERATE WITH CONTINUOUS

SIGNALS AND ARE USED TO CONSTRUCT ELECTRONIC CIRCUITS SUCH AS AMPLIFIER, VOLTAGE COMPARATORS, etc. DIGITAL INTEGRATED CIRCUITS OPERATE WITH BINARY SIGNALS AND ARE INVARIABLY CONSTRUCTED WITH ICs.

LOGIC GATES

ALL TYPES OF BOOLEAN  LOGIC GATES i.e AND, OR, XOR, XNOR, CAN BE CREATED FROM A SUITABLE NETWORK OF NAND OR NOR GATES.

DIGITAL INTEGRATED CIRCUITS

THE VARIOUS LOGIC FAMILIES CAN BE PLACED INTO TWO BROAD CATEGORIES ACCORDING TO THE IC FABRICATION PROCESS:-

1. BIOPOLAR

2. METALL OXIDE SEMICONDUCTOR

BIPOLAR LOGIC FAMILIES

THE IMPORTANT ELEMENT OF A BIPOLAR IC IS RESISTORS, TRANSISTORS, AND DIODES. BASED ON THE TWO MAIN OPERATIONS OF BIPOLAR ICs. i.e SATURATED AND NON-SATURATED, BIPOLAR ICs ARE CLASSIFIED AS:

1. SATURATED LOGIC

2. NON-SATURATED LOGIC

THE FOLLOWING ARE THE SATURATED BIPOLAR LOGIC FAMILIES:-

#RTL                              #DCTL (DIRECT-COUPLE TL)

#OTL                             #HTL (HIGH THRESHOLD LOGIC)

#TTL                             

THE FOLLOWING ARE THE NON-SATURATED LOGIC:-

1. SCHOTTKY TTL

2. ECL (EMITTER COUPLED LOGIC)

# MOS FAMILIES

THE MOS FAMILY INCLUDE:-

1. PMOS     P-CHANNEL MOSFETS

2. NMOS    N-CHANNEL MOSFETS

3. CMOS    COMPLEMENTARY MOSFETS

CHARACTERISTICS OF DIGITAL ICs

#SPEED OF OPERATION (PROPAGATION DELAY)

#POWER DISSIPATION (POWER CONSUMED)

# NOISE IMMUNITY

#OPERATING TEMPERATURE

#POWER SUPPLY REQUIREMENT

RESISTOR TRANSISTOR LOGIC (RTL)

RTL IS A CLASS OF DIGITAL CIRCUITS BUILD USING RESISTORS AS THE INPUT NETWORK AND BIPOLAR JUNCTION TRANSISTOR (BJTs) AS SWITCHING DEVICES. RTL NETWORK WAS WIDELY USED FOR DISCRETE MODULES BUT FORMED LESS SUITABLE IN INTEGRATED CIRCUIT FROM BECAUSE OF ITS LOW LOGIC LEVELS (ABOUT 1V) ON 3-4 VOLT SUPPLIES. IT HAS ALSO POOR FAN-OUT AND NOISE IMMUNITY. IT IS STILL BEING OF INTEREST FOR DISCRETE MODULE CONSTRUCTION. 

IN RTL THE RESISTOR INCREASE THE INPUT RESISTANCE REDUCE THE SWITCHING SPEED OF THE CIRCUIT THIS IS THE RISE AND FALL TIMES OF INPUT PULSE. AN EACH USED TO INCREASE THE SPEED OF RTL CIRCUIT IS MEET A SPEED UP CAPACITANCE IN PARALLEL WITH BASE.

THE FOLLOWING ARE THE CHARACTERISTICS OF THE RTL:-

1. SPEED OF OPERATION IS LOW, i.e THE PROPAGATION IS OF THE ORDER OF 500ns IT CAN'T OPERATE AT SPEEDS ABOVE 4MHZ.

2. POOR NOISE IMMUNITY

3. SENSITIVE TO TEMPERATURE.

DIODE TRANSISTOR LOGIC (DTL)

IN DTL THE LOGIC IS IMPLEMENTED USING DIODES AND TRANSISTORS. A BASIC CIRCUIT OF DTL LOGIC FAMILY.

EACH INPUT IS ASSOCIATED WITH ONE DIODE. THE DIODE AND 4.7K RESISTOR FROM AN AND GATE OF INPUT A, B, AND C ARE LOW THE CORRESPONDING DIODE CONDUCT CURRENT THROUGH 4.7K RESISTOR. THUS THERE IS NO CURRENT THROUGH THE DIODES CONNECTED IN SERIES TO THE RESISTOR BASE HENCE THE TRANSISTOR DOES'T CONDUCT THUS REMAINS IN CUTOFF AND OUTPUT IS HIGH.

IF ALL THE INPUTS A, B, C ARE DRIVEN HIGH, THE DIODE IN SERIES CONDUCTS, DRIVING THE TRANSISTOR INTO SATURATION. THUS OUTPUT IS LOW.

THE FOLLOWING ADVANTAGES AND DISADVANTAGES DTL OVER RTL:-

ADVANTAGES:-

1. FAN-OUT IS HIGH

2. NOISE IMMUNITY IS GOOD

 DISADVANTAGES:-

1. MORE ELEMENTS ARE REQUIRED

2. PROPAGATION DELAY IS MORE (TYPICALLY 3ons) AND THIS SPEED OF OPERATION IS LESS.

TRANSISTOR-TRANSISTOR LOGIC

TRANSISTOR IS THE BASIC ELEMENT OF THEIR LOGIC FAMILY WHICH OPERATES EITHER IN CUT-OFF OR SATURATED REGION. THE FIRST VERSION OF TTL IS KNOWN AS THE STANDARD TTL.

STANDARD TTLs ARE AVAILABLE IN VARIOUS OF:-

1. TTL WITH OPEN COLLECTOR OUTPUT

2. TRISTATE TTL