why there i sphase difference between input and output voltage in transistor (180)

because to pursue with active mode or during the time of amplication of a signal BE & CB junctions need to be biased as forward & reverse respectively in case of npn CE transistor..

by increasing Vi, Ib goes higher.. Ic goes much higher i.e. β times Ib where β is amplification factor, having a large value.. results drop across Rc, Ic.Rc

by applying KVL we get an eqⁿ Vi = Vo - Ic.Rc or, Vo = Vi - Ic.Rc

to obtain AC characteristics by applying an AC biasing makes drop across Rc greater value than i/p voltage Vi because of β.Ib

∵ Ic.Rc > Vi ∴ Vo must be -ve.. or, Vo = -Vi

i.e. Vcb > Vbb results reverse biasing goes superior than forward biasing..

so why, a 180⁰ phase shift takes place..