# What are Universal gates used for?

## Introduction

Apart from basic gates there are other two gates using which all complex digital circuits can be simplified.

Using these gates the basic gates also can be constructed.

The gates are NAND gate and NOR gate. These two gates are called universal gates.

#### NAND gate:

NAND stands for NOT AND.

The AND gate output is NOTed so it is called NAND gate.

When all the inputs of NAND gate are HIGH the output is LOW and for all other combination of inputs the output is HIGH.

NAND gate is also called bubbled OR gate, because if we invert the input of OR gate and then do the OR operation the outputs will be the same as the NAND gate.

#### Construction of NOT gate using NAND gate:

We can construct NOT gate using NAND gate.

When we short all inputs of a NAND gate into one input, it will act as a NOT gate.

The symbol and truth table are given below.

#### Construction of AND gate using NAND gate:

We can construct AND gate using NAND gate.

When we provide NAND gate output to a NAND gate inverter it will act as a AND gate.

The symbol and truth table are given below.

#### Construction of OR gate using NAND gate:

We can construct OR gate using NAND gate.

When we invert both inputs of a NAND gate using NAND gate inverter and supply it to a NAND gate the output will be same as OR gate.

The symbol and truth table are given below.

#### NAND gate using diode and transistor logic (DTL):

In the above circuit when A and B both inputs are 0V, D1 and D2 both diodes are forward bias. Current flows through D1 and D2, T remains off. Therefore the voltage drop across the collector of T appears across output.

When input A is 0V and B is +5V, D1 is on and D2 is off. Current flows through D1, T remains off. Therefore the voltage drop across the collector of T appears across output.

When input A is +5V and B is 0V, D1 is off and D2 is on. Current flows through D2, T remains off. Therefore the voltage drop across the collector of T appears across output.

When A and B both inputs are +5V, D1 and D2 both diodes are reverse bias. Current flows through transistor T, D1 and D2 remains off. Therefore there is no voltage drop across the collector of T so output is 0V.

#### NAND gate ICs:

 IC number Number of inputs Number of NAND gates 7400 2 4 7410 3 3 7420 4 2 7430 8 1

### NOR gate:

NOR stands for NOT OR.

The output of OR gate is NOTed.

When all inputs of NOR gate is LOW the output is HIGH, for all other combinations of inputs the output is LOW.

The NOR gate is also called bubbled AND gate. Because if we invert the inputs of AND gate and supply it to AND gate the output will be same as NOR gate.

#### Construction of NOT gate using NOR gate:

We can construct NOT gate using NOR gate.

When we short all inputs of NOR gate into one and conduct the operation it will act as NOT gate.

The symbol and truth table are given below.

#### Construction of AND gate using NOR gate:

We can construct AND gate using NOR gate.

When we invert the inputs of NOR gate using NOR inverter and do the NOR operation the output will be same as AND gate.

The symbol and truth table given below.

#### Construction of OR gate using NOR gate:

We can construct OR gate using NOR gate.

When we invert the output of NOR gate using NOR inverter and do the operation the output will be same as OR gate.

#### NOR gate using transistor logic:

In the above circuit when A and B both inputs are 0V, T1 and T2 both transistors are off. There is no current flow in the circuit. Therefore the voltage drop across the collector of T2 appears across output.

When input A is 0V and B is +5V, T1 is off and T2 is on. Current flows through T2. Therefore there is no voltage drop across the collector of transistor. So output is 0V.

When input A is +5V and B is 0V, T1 is on and T2 is off. Current flows through T1. Therefore there is no voltage drop across the collector of transistor. So output is 0V.

When A and B both inputs are +5V, T1 and T2 both transistors are on. Current flows through transistor T1 and T2. Therefore there is no voltage drop across the collector of transistor so output is 0V.

#### NOR gate ICs:

 IC number Number of inputs Number of gates 7402 2 4 7427 3 3 7425 4 2

So this is all about universal gates.

The cost of a digital system can be reduced by reducing number of gates. Therefore the universal gates are made for. Most digital systems use only one type of gate either NAND or NOR to produce an economic system.

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