Why is MATLAB giving me the wrong Bode phase plot?

Question

Gongwei Chen 2019-11-10

回答： Urmila Rajpurohith 2019-11-14

I plotted

using

Z=[];
P=[1,-1];
K=100
G=zpk(Z,P,K);
G
bode(G)

And got

But it should be 0 deg not -180.

Answer 1

Urmila Rajpurohith 2019-11-14

The Bode Phase plot which you got is correct because the transfer function is written as

G=(-100)/((1+jw)(1+(-1)jw))

When you calculate phase angle then

For (1+jw) the phase angle is= -atan(w)

For (1+(-1)jw) the phase angle is which is -atan(-w) written as atan(w)-180

So, the overall phase angle becomes ɸ =-atan(w) + atan(w)-180

ɸ = -180

So, the Phase plot you will get at -180 deg.Here atan is nothing but inverse of tan.