Question

why in common emitter output wave is 180 out of phase?

Answer 1

The common emitter mode, taking output from collector, actually has a positive current gain which causes the drop across the load to increase thereby decreasing the voltage at the collector. The above uses the standard convention that input/output currents and considered positive into their respective node.
If you take the output across an emitter resistor in this mode the output is in phase with the input, usually with a gain close to one.

Answer 2

for CE we can write the KVL at output as
V0 = VCE = Vcc - IcRc
eliminated the emitter Resistance
No Ic = B Ib
where B = beta gain
So
differentiate the output KVL we get
dV0 = -dIc Rc = -dIb B Rc
as B and Rc are assumed constant for our CE circuit
The - sign indicates that phase of output voltage is 180 deg out of phase with Input current. Input current being function of input voltage, u get Input voltage and Output voltage are 180 out of phase. This is the mathametical explanation....

Answer 3

For more Information....

http://www.ask.com/faqcentral/PHASE_180.html

Thank you