Blog 2: Bipolar Junction Transistor

Transistor is a semiconductor device,which can be used for switching and amplifying signal and power.

Transistor is a alterable switch controlling output current based on input voltage,so it can be used as a current switch.It is different than the normal mechanical switch and relay as the transistor is using signals to control and the switching frequency is way faster

When a voltage or current apply to the transistor,the controlled output power can be much more than the controlling input power,so a transistor can be using to amplify a signal.


There are two main types of transistors,one is BJT(Bipolar Junction Transistor) and the other one is FET(Field-effect Transistor)
A BJT normally has three terminals which are Collector(C),Base(B) and Emitter(E).A BJT is a combination of two junction diodes,when a p-type semiconductor is in the middle of two n-type semiconductor it is a NPN transistor,or when a n-type semiconductor is in the middle of two p-type semiconductor it is a PNP transistor.

This picture shown below is a three terminal bipolar transistor.

How to identify the terminals and NPN or PNP of a transistor?
First,get a multimeter and set to diode test, here we label the 3 terminals as leg1,leg2 and leg3 from left to right.Now test the continuity between legs in order of :
1 to 2
1 to 3
2 to 1
2 to 3
3 to 1
3 to 2

Whichever leg makes a reading on the multimeter with the other two is the base,if the Base is connected with the negative probe it is a PNP transistor,if the Base is connected with the positive probe it is a NPN transistor and whichever has more reading is Collector and the other one is Emitter.

The most BJT used today is NPN transistor as it produce greater current and faster operation so I am only going to take about NPN Bipolar Junction Transistor.


In a NPN transistor,the current path from the Collector to Base is in fact a reverse biased diode,and the current path from the Base to Emitter is in fact a forward biased diode.For the current to pass from Base to Emitter the voltage needs to exceeds 0.7V and when this voltage reaches more than 0.7V, the Collector current will flow.

A bipolar transistor normally has three distinct regions of operation which are Active, Saturation and Cut off as photos shown below.

Active: is an operation region when transistor is amplifying small signals
Saturation: is an operation region when both junction diode are forward biased so it will generates high current flow through Emitter from Collector,this corresponds a logical "on" or closed switch.
Cut off: is an operation region when both junction diode are reverse biased which it is the opposite of saturation.This is result in very little current which corresponds a logical "off " or opened switch


When amplification small signals the transistor is operating in the active region and it will make high power dissipation as it drops half the voltage across Vce.

The maximum power a transistor can dissipate is known as Pd which equals to I x V according to ohm's law.

The ratio between Collector current and Base current is know as the current gain Beta where Beta equals to Ic/Ib, this current gain is a major advantage of the transistor and can be used to all kinds of application.


The other ability of the transistor is using as a switch to turn things on and off which can often be seen in automotive applications such as Injectors and Ignition Coils

The Collector current is proportionally limited by its Bass current, so a switching transistor is actually a current-controlled switch. When there is a voltage supply which exceeds more than 0.7V to Base, the current will flow through from Collector to Emitter(Vce) and it's output voltage and speed changes proportionally when  the Base voltage and frequency changes.

What could cause a bad transistor?

* Wrong resister is been used in series with base

* Too much current going to Collector

* No current going to Base

* No supply voltage

* Circuit connected to different ternimals

This is just some typical reason that could cause a transistor go bad, there are more reason that we can trouble-shotting in specific case

Transistor is the most important component in any circuit.A bad transistor could cause the device that it is swithcing or amplifying for not working or working as it should be,sometimes it can destory the device which causes big money mistakes.










References:


auto101.com
wikipedia
Unitec Moodle
Vijay Naidu