LEARNING AND UNDERSTAND TRANSISTORS

 Introduction
A semiconductor device with three connections, capable of amplification in addition to rectification.Transistors are used most widely used in most of the electronic projects and circuits. Transistors are used for amplification , like amplifying small current output of any logic IC to drive a lamp or relays etc.
A transistor may be used as a switch (either fully on with maximum current, or fully off with no current) and as an amplifier (always partly on).

The amount of current amplification is called the current gain, symbol hFE.


Types

There are two types of standard type of transistors, NPN and PNP, with different circuit symbols. The letters refer to the layers of semiconductor material used to make the transistor. Most transistors used today are NPN because this is the easiest type to make from silicon. If you are new to electronics it is best to start by learning how to use NPN transistors.

The leads are labelled base (B), collector (C) and emitter (E).
These terms refer to the internal operation of a transistor but they are not much help in understanding how a transistor is used, so just treat them as labels! In addition to standard (bipolar junction) transistors, there are field-effect transistors which are usually referred to as FETs. We will discuss about them in any other article.
 

How to connect a transistor in circuit

Transistors have three leads which must be connected the correct way round. Please take care with this because a wrongly connected transistor may be damaged instantly when you switch on.
If you are lucky the orientation of the transistor will be clear from the PCB or strip board layout diagram, otherwise you will need to refer to a supplier's cataloge to identify the leads.
The drawings on the right show the leads for some of the most common case styles. Please note that transistor lead diagrams show the view from below with the leads towards you. This is the opposite of IC (chip) pin diagrams which show the view from above.

Soldering

Transistors can be damaged by heat when soldering so if you are not an expert it is wise to use a heat sink clipped to the lead between the joint and the transistor body. A standard crocodile clip can be used as a heat sink.
Do not confuse this temporary heat sink with the permanent heat sink (described below) which may be required for a power transistor to prevent it overheating during operation.

Testing a transistor

Transistors can be damaged by heat when soldering or by misuse in a circuit. If you suspect that a transistor may be damaged there are two easy ways to test it.

1- Testing with a multimeter

Use a multimeter or a simple tester (battery, resistor and LED) to check each pair of leads for conduction. Set a digital multimeter to diode test and an analogue multimeter to a low resistance range.

Test each pair of leads both ways (six tests in total)

• The base-emitter (BE) junction should behave like a diode and conduct one way only.
• The base-collector (BC) junction should behave like a diode and conduct one way only.
• The collector-emitter (CE) should not conduct either way.

The diagram shows how the junctions behave in an NPN transistor. The diodes are reversed in a PNP transistor but the same test procedure can be used.  

2. Testing in a simple switching circuit





Connect the transistor into the circuit shown on the right which uses the transistor as a switch. The supply voltage is not critical, anything between 5 and 12V is suitable. This circuit can be quickly built on breadboard for example. Take care to include the 10k resistor in the base connection or you will destroy the transistor as you test it!
If the transistor is OK the LED should light when the switch is pressed and not light when the switch is released.
To test a PNP transistor use the same circuit but reverse the LED and the supply voltage.Some multimeters have a 'transistor test' function which provides a known base current and measures the collector current so as to display the transistor's DC

current gain hFE.


Transistor codes

There are three main series of transistor codes used in the :
            Codes beginning with B (or A), for example BC108, BC478

• B is for silicon, A is for germanium (rarely used now). The second letter indicates the type; for example C means low power audio frequency; D means high power audio frequency . The rest of the code identifies the particular transistor. There is no obvious logic to the numbering system. Sometimes a letter is added to the end (eg BC108C) to identify a special version of the main type,

• Codes beginning with TIP, for example TIP31A 

TIP refers to the manufacturer: Texas Instruments Power transistor. The letter at the end identifies versions with different voltage ratings.
Codes beginning with 2N, for example 2N3053
The initial '2N' identifies the part as a transistor and the rest of the code identifies the particular transistor. There is no obvious logic to the numbering system.

Use of Transistor as an amplifier

Transistor are used to amplify small inputs to use them in various electronic applications. to use transistor as an amplifier just connect the transistor in the circuit as shown. now provide small input at input terminal and measure the output at output end with the use of multimeter and you will found that the output is amplified.

 To maximize the current gain (Darlington Pair)


This is two transistors connected together so that the amplified current from the first is amplified further by the second transistor. This gives the Darlington pair a very high current gain such as 10000. Darlington pairs are sold as complete packages containing the two transistors. They have three leads (B, C and E) which are equivalent to the leads of a standard individual transistor. You can make up your own Darlington pair from two transistors.

For example:

• For TR1 use BC548B with hFE1 = 220.
• For TR2 use BC639 with hFE2 = 40. 
• The overall gain of this pair is hFE1 × hFE2 = 220 × 40 = 8800.