Hi,

Why does a basic 2 BJT differential amplifier has phase shift of about 180 degrees between its input and output ?

Thanks,

 

because of the type of circuit. It has emitters in common and output on the collector. This is an inverting amp configuration. The output will always be 180 out from the input on the base.

More stuff on BJT diff amps
http://users.ece.gatech.edu/mleach/ece3050/notes/bjt/bjtdiffamp.pdf