Transistor Circuit Configurations

A transistor circuit has only three terminals but needs both an input and an output loop, so one terminal must necessarily be shared (common) between the two loops -- this gives exactly three possible circuit configurations, named after whichever terminal is the common one: common base, common emitter and common collector, covered in the three sub-sections below. Whichever terminal is chosen as common still carries current in both loops -- it is never simply grounded out of the circuit -- so the choice changes which two currents count as 'input' and 'output' for gain and impedance purposes, and this single choice is what gives the three configurations (common base, common emitter, common collector) their very different in …