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Why is MATLAB giving me the wrong Bode phase plot?

Asked by Gongwei Chen on 10 Nov 2019 at 15:54
Latest activity Answered by Urmila Rajpurohith on 14 Nov 2019 at 10:26
I plotted
demo.png
using
Z=[];
P=[1,-1];
K=100
G=zpk(Z,P,K);
G
bode(G)
And got
demo.png
But it should be 0 deg not -180.

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1 Answer

Answer by Urmila Rajpurohith on 14 Nov 2019 at 10:26

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.

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