Types of transistor

There are two main types of transistors-junction transistors and field effect transistors. Each works in a different way. But the usefulness of any transistor comes from its ability to control a strong current with a weak voltage. For example, transistors in a public address system amplify (strengthen) the weak voltage produced when a person speaks into a microphone. The electricity coming from the transistors is strong enough to operate a loudspeaker, which produces sounds much louder than the person's voice. 

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Now answer the questions and add these to the comment page below the post.

1. What are the main types of transistor ?
2. What are the different between NPN

 and PNP ?

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Transistor