explain the working of a common emmiter npn transistor amplifier?
A transistor consists of three regions of doped semiconductors called: emitter, base and collector. It is either an N type semiconductor is sandwiched between two P type semiconductors or a P type semiconductor is sandwiched between two N type semiconductors. The emitter is highly doped, the collector is moderately doped and the base is lightly doped.
In a common emitter configuration, the base emitter junction is kept as input and is forward biased. While the collector-emitter is the output and it is reversed biased. A small current signal in the base can be used to control a large current flowing between the emitter and the collector. So, a small signal at the input can be magnified to a large signal at the output. This is how a transistor is used as an amplifier.
In an NPN transistor, in common emitter configuration, the input signal is given to the base. And in the positive half of the input signal, large current flows from the collector to the emitter through the base. Since, the base is lightly doped a very small current completes the base-emitter loop. The input signal is replicated in the output with higher amplitude.
The basic biasing circuit for a common emitter mode. Here, IE = IB + IC
For mathematical understanding and derivations you may go through the study material.