Looking at the DC behavior of the almlifier and following the convention that lower case letters represent changes in the respective quantities:
The gain is
which for small rE can be approximated by
but for AC signal variations for which CE is an effective bypass, the gain is
With the bypass capacitor in place, the voltage gain is dertemined by the collector resistor RC and the effective resistance of the transistor rE.
This effective resistance is
where RBE is the forward resistance of the base-emitter diode.