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A Practical Introduction to Transistors

written by: Swagatam • edited by: Swagatam • updated: 2/25/2014

Basically the function of a transistor is to amplify a small 'base/emitter' voltage into a much larger 'collector/emitter' voltage. A simple experiment will explain how does a transistor work.

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    Transistors were a revolution, when they were first developed and introduced way back in 1930 s. They immediately replaced the cumbersome valves and the triodes in electronic circuits. Since then till date they are produced and used extensively. These tiny three-footed parts can be considered as the stepping-stone in the field of electronics.

    In this article we will try to understand how does a transistor work.

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    What are Transistors


    Just like diodes these too are active semiconductor components, which are available in both silicon and germanium types.

    These can be fundamentally classified into NPN and PNP categories. We wont discuss its internal composition or the doping details, since the explanation can be too vast to be confined in a single article. In simple words a NPN transistor will function with respect to the ground and accept a positive voltage as a trigger, whereas a PNP type will work with respect to positive supply and will be triggered with a negative voltage.

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    How do they Function

    The above functioning can be explained using a 'lever' analogy. We all know how a small force when applied to one end of a pivoted lever can be used to displace heavy objects. In an electronic circuit, transistors behave the same way i.e. a small current when applied to one of its three terminals ( the base ) a relatively huge current is allowed to flow through the remaining two leads ( from Collector to Emitter ).

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    Try it Yourself

    How Does a Transistor Work, Circuit Diagram, Image  

    The above functioning can be witnessed practically by doing the following experiment:

    • Take a general purpose NPN transistor for eg: BC547B, keeping the printed side towards you, the right and the left hand side leads are identified as the 'Emitter' and the 'Collector' respectively, the centre lead is obviously the 'Base'.
    • Connect ( by soldering ) the cathode ( short lead ) of an LED to the 'Collector', also connect a resistor of say 150 Ohms to anode ( long lead ) of the LED.
    • Join a resistor of around 10 K to the 'base' of the transistor.
    • Finally connect a positive supply ( 3 to 12 volts ) to the free end of the 150 Ohms resistor, and the negative to the 'Emitter' of the transistor.
    • Dont get confused if you find the LED not glowing, because this is where the actual test begins.
    • As per the Fig. the LED can be switched ON and Off by applying a small voltage (>0.6 ) to the free end of the 10 K resistor or the 'Base' of the transistor.
    • For testing purpose you can use the positive supply itself to toggle the LED. Though this voltage may look quite large, actually a very small part of it is reaching the 'Base', due to the 10 K current limiting resistor.
    • This resistor should be dimensioned according to the 'collector' load ( an LED in this case ), for a correct operation of the circuit, ( the formula can be obtained from one of my previous articles, ' current through LEDs...').
    • If a PNP transistor is used for the above experiment, just reverse the polarity of the LED and the supply voltage.

    The above experiment must have provided you a clear picture regarding how does a transistor work.

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