When a transistor is used in common emitter configuration the phase difference between input signal and output voltage is

V0=VCC−(IC+ic)RC⇒ ΔV0=−RC×Δic⇒ ΔV0=−RC×βΔiβBut ΔVS=Ri×ΔiB∴ ΔV0=−βRCRi×ΔVSWhen ΔVS is positive ΔV0 is negative and When ΔVS is negative, ΔV0 is positive.