NOT GATE (INVERTER LOGIC) using transistors

INTRODUCTION

A NOT gate is one of the basic logic gates in digital electronics.It is also called as an inverting gate or a negating gate.It negates the given input.If we apply logic 1 (usually 5 volts or sometimes above 2.5 volts is set as logic 1) as the input, then the gate switches to logic 0(usually 0 volts or sometimes below 2.5 volts is set as logic 0) as output , and vice-versa.The operation of this gate is pretty simple and very easy to understand. Besides being simple, the circuit is used in many applications.

TRUTH TABLE

The truth table of the NOT gate is shown in the following table.Its easy to understand as it has only two possible inputs with corresponding outputs.

                        

CIRCUIT BUILDING

CIRCUIT DIAGRAM : 

The circuit to realize this gate using transistors is very simple.We just require an 'npn' transistor and two resistors to build a basic working NOT gate along with a power supply.To observe its working in a more liberal way, we could connect an LED at its output as an indicator. We could use an npn transistor(like BC 547).The following figure shows the circuit diagram.

[circuit diagram]

The R1(330K) resistor is acting as the input resistor and it switches the transistor ON and OFF.When the transistor is ON by making the input high, the transistor switches the ground potential at  the emitter terminal to the collector , which is the output terminal.If the input is low , then the transistor is OFF , and the output at the collector terminal provides the supply voltage ( here 5 volt) through the collector resistor R3 , which in turn drives the LED to the ON state.An additional 1Mega ohms(R2) is provided at the base terminal of the transistor , inorder to avoid any error conditions when the input terminal is kept open or not connected state. Eventhough it is irrelevant in this type of single transistor circuits , it becomes helpful in error free operations  when we couple many number of transistors together. 

BREADBOARD  CONNECTION : 

We could use an npn transistor(like BC 547).The pin configuration assumed in the figure is CBE(collector,base,emitter) in the front view.The red line implies positive rail and the blue line implies negative rail in the breadboard.

1. At first , the input is given zero(to ground) or LOW state

                         The output will be high and the LED will glow.This is illustrated in the figure below.

2. Next , the input is given 5V(to positive rail) or HIGH state

                      The output will be low and the LED is driven into OFF state.This is illustrated in the firgure below.

This way, we can implement the basic logic gate in digital electronics - The NOT gate.

PRACTICAL OBSERVATIONS

When we set the base resistor as 330K , the  base current drawn is approximately 13 micro ampere when input is applied HIGH logic. At this stage, the transistor in ON and the collector current is about 1 milli ampere.This gives the gain of the transistor at about 76.9.The collector-emitter voltage is 0.1 volt , when the transistor is ON.It shows an open voltage of 5 volt when the transistor if OFF.

To know some basics of transistors, please read this article : https://www.instructables.com/id/Be-OBSESSED-With-Basic-Electronics/

That's all for this discussion.Hope you have enjoyed!!!

Next time we could make a latch circuit using transistors and try to have some fun with it.

Thank you!!!!