Question { 20087 }

how there is a 180 degree phase shift in output signal of CE
amplifier?

Answer

Draw the circuit of a common emitter amplifier, and see
what happens when base voltage is increased from 600mV to
602.6mV. If npn collector is returned to 15V through a 10k,
and initial current was 1mA, collector voltage would have
been 5V. When VBE becomes 602.6mV, current will increase in
base as well as collector. The current now becomes 1.1mA,
and drop in 10k now is 11V. This brings down the voltage at
collector to 4V, one volt LESSER than what it was earlier.
This makes the amplification phase reversed.