# need help in understanding with feedback command

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shipra le 23 Mai 2023
Commenté : shipra le 24 Mai 2023
I am using these commands to develop my transfer function with state space matrix, using pidtune to develop closed loop system
sys=ss(A_b,B_b,C_b,D1);
TFbuck=tf(sys);
C = pidtune(TFbuck,'PID')
These are the three commands that I have tried for closed loop controller. syst11 and syst1 are same (default is a negative feedback loop, or we can specify -1). But the first command, syst gives me different transfer function as compared to syst11, syst1. What is the difference in computation that gives different results.
syst = feedback(TFbuck*C,1)
syst11 = feedback(C,TFbuck,-1)
syst1 = feedback(C,TFbuck)
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### Réponse acceptée

Paul le 23 Mai 2023
Hi shipra,
In accordance with the the doc page feedback, the first argument is the forward path gain and the second argument is the feedback path gain, the third argument is the sign of the feedback, default is -1. So, we have mathematically
syst: TFbuck(s)*C(s)/( 1 + 1*TFBuck(s)*C(s) )
syst11 = C(s)/( 1 + TFBuck(s)*C(s) )
syst1 = C(s)/( 1 + TFBuck(s)*C(s) )
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shipra le 24 Mai 2023
Hello Paul,

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