Q.What is the phase difference between input and output in CE amplifier?

In a common-emitter (CE) amplifier the input signal is applied between base and emitter and the output is taken between collector and ground; the emitter is common to both, so it is also called a grounded-emitter amplifier. Voltage-divider bias ($R_1, R_2$) with a collector resistor $R_C$ and an emitter resistor $R_E$ sets the operating point.

Working: on the positive half-cycle the forward bias on the base-emitter junction increases, so $i_B$ and hence $i_C = \beta\, i_B$ increase; the drop $i_C R_C$ rises, and since $V_{CE} = V_{CC} - i_C R_C$, the collector (output) voltage falls. A positive input thus produces a negative output, giving a 180 phase reversal between input and output. …