Page 38 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Transistors

Objectives: At the end of this lesson you shall be able to
• explain the construction of bipolar transistors
• explain the classification and working of PNP and NPN transistors
• state the important packages and type number systems of transistor
• explain the methods of testing transistor.

Introduction: Transistor is an active device which can be compared to the heart of modern electronics. It
accepts small electrical signal either in the form of current or voltage at the input and then amplifies (increase
the amplitude) and provides a large signal at the output as in Fig 1. Transistors are used in almost all electronic
gadgets such as radio, TV, tape recorder, computer etc.,

Fig 1

Before the transistors were invented (1947), certain devices are used known as vaccum tubes or valves which
were used in amplifiers.
Compared with the present day transistors the vacuum tubes were big in size, consumed more power, generated
lot of unwanted heat and were fragile. Hence vaccum tubes became obsolete as soon as transistors came to
market.
Transistors were invented by walter H. Brazil and John Barlow of Bell Telephone Laboratories on 23rd Dec. 1947.
Compared to vaccum tubes transistors have several advantages. Some important advantages are listed below.
• Very small in size
• Light in weight

• Minimum power loss in the form of heat
• Low operating voltage
• Rugged in construction
• Long life and cheap.

To satisfy the requirements of different applications, several types of transistors in different types of packaging are
available. As in diodes, depending upon the characteristics, transistors are given a type number such as BC 107,
2N 6004 etc., The characteristics data corresponding to these type numbers are given in Transistor data books.
Transistor are available as bi polar, field effect and unijunction etc.,
A bipolar junction transistor uses two opposite polarity of doped semiconductor i.e. ‘N’ type and ‘P’ type.
A field-effect transistor uses electrostatic field of charged carriers for its working.

An unijunction transistor uses a single junction of ‘P’ and ‘N’ type semiconductor.

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CITS : Power - Electrician & Wireman - Lesson 60-69