diary on
s=tf('s')
 
Transfer function:
s
 
P=40/(s^2+15*s+50)
 
Transfer function:
      40
---------------
s^2 + 15 s + 50
 
K=61.25

K =

   61.2500

H=(K*P)/(1+K*P)
 
Transfer function:
       2450 s^2 + 36750 s + 122500
------------------------------------------
s^4 + 30 s^3 + 2775 s^2 + 38250 s + 125000
 
step(100*H)

K=50

K =

    50

H=(K*P)/(1+K*P)
 
Transfer function:
       2000 s^2 + 30000 s + 100000
------------------------------------------
s^4 + 30 s^3 + 2325 s^2 + 31500 s + 102500
 
step(100*H)

K=100

K =

   100

H=(K*P)/(1+K*P)
 
Transfer function:
       4000 s^2 + 60000 s + 200000
------------------------------------------
s^4 + 30 s^3 + 4325 s^2 + 61500 s + 202500
 
step(100*H)
diary off