Preamplifiers are designed to deliver the needed current/voltage gain to a Power Amplifier (which I'll explain in a different tutorial). There are 3 distinct configurations to do that. (from left to right) Common collector, common emitter,common base.
-Common Collector - typically has a voltage gain below 1 but with a few readjustments can have some voltage gain no more then 20-30. This preamp specifies itself with a high current gain and its purpose is to act as a buffer between unmatched stages by having a high input impedance and low output one.
-Common Emitter - this preamp works in a class A mode but since its a preamp the efficiency should not bother us. It has a medium high and output impedance. Has a very high voltage gain and could have a high current gaim too but that would require more power and since it's in a class A mode the efficiency levels will drop sharply with the increaae of power. Simce this amp has a cap for more gain the signal becomes more distorted and ive decided to add a simple negative feedback to negate some of the distortion.
-Common base- Now this topology is a little more difficult and is not widely used because it typically has a very low input impedance and the fact that high amounts of DC passes through the input source. This topology is mostly used in high frequency amplifiers because it has the best frequency response characteristics from the 3 topologies. The gain may be limited by either decreasing the base dividers capacitor value. It doesn't provide current gain comparing input/output but typically has very high voltage gain restricted only by the transistor hfe.
