There are three configurations. They are (1) Common Base (2) Common emitter (3) Common collector.
(1) Common Base configuration `:`
In this configuration base is earthed. Base is common both input and output. Input is given across base-emitter and output is taken across base-collector.
(2) Common Emitter configuration `:`
In this configuration emitter isearthed. Emitter is common both input and output . Input is given across base-emitter and output is taken across collector-emitter.
(3) Common collector configuration `:`
In this configuration collector is earthed. Collector is common both input and output. Input is given across base collector and output is taken across collector-emitter.
