The amplifier is probably the single most important composite circuit element in electronics. Certainly such familiar devices as radios and music systems would be impossible without amplifiers. The very large majority of amplifiers these days are solid state, that is, they use transistors. While there are many types of transistors, the first mass-produced transistors were BJTs, or bipolar junction transistors, and understanding transistor amplifiers, for most people, begins with these.
The transistor has three terminals and can be arranged in three basic amplifier configurations, the common emitter, common base (or grounded base) and common collector or emitter follower amplifiers.
Common base amplifiers are not commonly employed at lower frequencies as, amongst other reasons, they have a low input impedance, although they can be found in amplifiers for e.g. moving coil microphones. They are sometimes employed as current buffers, having a current gain of 1,...