Uses of Logic gates (basic gates)

#Technical studies – Study 6

Introduction

Logic gates are the fundamental building blocks of digital circuit. The term logic gate defines that the circuit that able to take decisions.

Logic gates give one type of output when certain types of inputs are present and another type of output when some other types of inputs are present.

There are three basic types of gates present in digital system. Those are AND gate, OR gate and NOT gate or INVERTER.

The gates are electronics devices that are made from many electronics components by very large scale integration or large scale integration or small scale integration.

Logic gates work in two levels that is 0 and 1 or HIGH and LOW or ON and OFF or TRUE and FALSE.

The table that represents the input levels and the respective output levels is called truth table.

Universally most digital devices take level 1 as +5 V and level 0 as 0V.

AND gate:

AND gate takes two or more inputs and provides one output.

When all the inputs to the AND gate is high the output is high and even if one input is low the output is low. So the AND gate is called all or nothing gate.

 The symbol for AND operation is (.).

AND gate symbol and truth table

AND gate using diode logic:

 The working of AND gate can be understood using diode logic.

AND gate using diode logic

In above circuit when A and B both inputs are LOW, both diodes D1 and D2 are in forward bias. Current flows through both diodes, so output becomes LOW.

When input A is LOW and B is HIGH, diode D1 is forward bias and diode D2 is reverse bias. Current flows through D1 and D2 is off, output becomes LOW.

When input A is HIGH and B is LOW, diode D1 is reverse bias and diode D2 is forward bias. Current flows through D2 and D1 is off, so output becomes LOW.

When A and B both inputs are HIGH, both diodes D1 and D2 are reverse bias. Current flow occurs through R and there is no voltage drop across R, so +5V appears across output.

AND gate using Transistor logic:

AND gate using transistor logic and truth table

In the above circuit when A and B both inputs are LOW, both transistors T1 and T2 are off and T3 is on. Current flows through T3, so output becomes LOW.

When input A is LOW and B is HIGH, transistor T1 is off, T2 is on and T3 is on. Current flows through T3, so output becomes LOW.

When input A is HIGH and B is LOW, transistor T1 is on, T2 is off and T3 is on. Current flows through T3, so output becomes LOW.

When A and B both inputs are HIGH, transistor T1 is on, T2 is on and T3 is off. Current flow occurs in T1 and T2. T3 is off, so +5V appears across output.

AND gate IC numbers:

IC number Number of inputs Number of gates
7408 2 4
7411 3 3
7421 4 2

OR gate:

OR gate takes two or more inputs and produces one output.

When any of the inputs is HIGH the output is HIGH and if all inputs are LOW the output is LOW. So OR gate is called any or all gate.

The symbol for OR operation is (+).

OR gate symbol and truth table

OR gate using diode logic:

The working of OR gate can be understood using diode logic.

OR gate using diode logic

In the above circuit when A and B both inputs are LOW, both diodes D1 and D2 are off. There is no flow of current in the circuit, so the output becomes LOW.

 When input A is HIGH and B is LOW, diode D1 is on and diode D2 is off. Current flows through D1 and R, so output becomes HIGH.

When input A is LOW and B is HIGH, diode D1 is off and D2 is on. Current flows through D2 and R, so output becomes HIGH.

When A and B both the inputs are HIGH, both diodes D1 and D2 are on. Current flows through D1, D2 and R and output becomes HIGH.

OR gate using transistor logic:

OR gate using transistor logic and truth table

In the above circuit when A and B both inputs are LOW, transistor T1 is off,T2 is off and T3 is on. Current flows through T3, so the output is LOW.

When input A is LOW and B is HIGH, transistor T1 is off, T2 is on and T3 is off. Current flows through T2, T3 is off so there is a voltage drop across collector of T3 that +5V appears across output.

When input A is HIGH and B is LOW, transistor T1 is on, T2 is off and T3 is off. Current flows through T1, T3 is off so there is a voltage drop across collector of T3 that +5V appears across output.

When A and B both inputs are HIGH, transistor T1 is on, T2 is on and T3 is off. Current flow occurs in T1 and T2. T3 is off so there is a voltage drop across collector of T3 that +5V appears across output.

7432 is a two input OR gate IC and have four OR gates in it.

NOT gate:

NOT gate is also called as inverter. It has one input and one output.

When the input is LOW the output is HIGH and when the input is HIGH the output is LOW.

The symbol for NOT operation is (-) bar.

NOT gate symbol and truth table

Inverter using Transistor logic:

NOT gate using transistor logic

In the above circuit when input A is LOW, transistor T1 is off so the applied +5V appears across output.

When input A is HIGH, transistor T1 is on. Current flows through T1 so output becomes LOW.

7404 is a inverter IC and have six inverters in it.

Please write in the comment box below if you have any questions.

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