what is a transistor? use circult diagram as an aid to distinguish between npn and pnp transistors?

Question

when they are used in common emitter modes. and why are npn transistor preffered in high-sped circult.

Answer 1

Here is a wikipedia article that shows the schematic symbol for the 2 types of transistors.

http://en.wikipedia.org/wiki/Bipolar_junction_transistor

The article also explains a little bit about them.

Common emitter amplifiers (as opposed to common base or common collector) have good characteristics for an amplifier. This configuration has high input impedance, relatively low output impedance, and high voltage gain.

NPN transistors are preferred, because the N-type material has electrons in the silicon crystal lattice as the 'carrier' of the majority current (that which is amplified), while in P-type material has a lack of electrons, called 'holes' that are the majority carrier. Electrons are inherently faster than 'holes' for carrying the amplified current in the transistor, so NPN types are inherently faster. The "P" in an NPN is the very thin base layer -- it's the collector and emmiter that are the N type material.

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