TRANSISTORS & DIODES:

consists of 4 main operations
1. Diodes & Transistors
2. Rectifiers 
3. Regulators
4. Op-Amps
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Transistors:

An amazing comparison to transistor current flow was given by an ole southern teacher of mine.  Professaa' Hayes would say tha transistors work like the ole water pump out in the back yard.  You had to get a cup of water to pour down in the pump to prime the pump.  This is what the base current was like, the prime water. Once the the prime water went down in the pump, it seemed to suck the fresh water up out the ground and the water would have a generous flow, cool and delicious.  Once the base current primed the emmitter-collector current to start flowing, the excessive electrons would overflow from the base into the collector, because they are attracted to the positive bias voltage. Then the in phase collector current would have the same ups and downs that the base curren has but just much greater.  Thus, the current is amplified.  Although not all transistors amps have in-phase collector currents.

Transistors are used in 3 main configurations.  These are common base, common collector, and common emitter, which is the most popular.  Properly (powered) biased transistors can amplify input signals connected to the base of the transistors.  This input signal changes the resistance of the collector-emitter voltage, thus changing the out voltage, current or power gain.  There are several different classes of transistors having different distortion levels, different efficiencies, power setup or biases, and different uses.  They may be used for class C, radio frequency amplification, large power applications as classes B and AB amps, and small signal, class A amps.  Usually there are more than one transistor or transistor stage in an amp.  These stages are connected either directly, or with a coupling capacitor or a transformer.  Transformer coupled amps provides impedance matching, high efficiency and tuneablilty while only direct coupled amps respond to direct current or DC and capacitor coupled amps are the easiest to use.