Common Emitter amplifier

[Khan740]

Hi,
Why there is phase change in Common Emitter Amplifier?

 

[Syncopator]

There isn't. There is an inversion.

When the base of an NPN transistor goes positive the base-emitter current increases, and so, therefore does the collector current.

The increase in collector current produces a larger voltage drop across the collector load. The collector voltage therefore goes less positive.

And vice versa for a negative change at the base.

In a PNP transistor the action is similar, but the polarities are opposite to the NPN case.