TRANSISTOR

 The transistor has been around for a fair few years now, first started out
 as a smaller replacement for the valve more economical, the transistor is
 found almost everywhere.
 Actually the transistor was invented way back in 1952.
 There are many many types of Transistor for a whole range of differant
 applications many now are of a Silicon based material, where as a few years
 age it used to be all Germaniun but Silicon has the edge over Germanium.
 There are such things as NPN, PNP, MOSFET, Silicon, Germanium, Low power,
 Medium power, High power, GASfet, to mention but a few.
 If you take a look at 'Maplin Electronic' catalogue you see just how many
 there are, and even then that is not all of them.

 You must try to remember that NPN is of course the exact opposite to PNP
 so NPN means Negative posative Negative, and PNP means Posative Negative
 Posative.
 To get a clear idea of what I mean look at the IFF 'NPNPNP' because the
 device symbols are NOT the same.

 There are such things as base I.D such as Base Collector Emiter or if we
 consider the MOSFET then we might expect to see Ground Gate Source
 so it is veried to say the least, look at IFF 'Base I.D' to see what I
 mean.
 The base configurations are viewd from underneath.
 A transistor will mainly amplify a current to a larger current, so if we
 had a crystal set radio the sound will be just about audible through a
 crystal ear piece but if we add a transistor amplifier to the set then the
 sound would be far more audible.
 Look at the IFF's 'bc108' and 'bc109' for an example of a amplifier.
 But transistors are not just amplifiers they can be used in circuits such
 as switch circuits or power supplies to control a voltage via a zener
 diode.
 A transistor can be made to switch on and off by applying a small amount of
 current to the base of the transistor.
 When there is no current at the base, then no current can flow between the
 collector and the emitter, and so the transistor is off.
 A small current at the base enables the transistor to conduct the current
 from the collector to the emitter, and allows a larger current to flow
 through the transistor.
 Components connected to the collector can use this larger current.

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