I'm analyzing open loop stability of an amplifier. Plotting the bode diagram I can see a negative phase margin (PM) indicating the system is not stable. When I plot the pole/zero plot however all the poles still remain on the left half plane. See attached figure. Note that instability results due to the 3rd zero crossing where the PM is negative. I do not understand why the complex poles have not shifted to right half plane (RHP). Using SPICE however I can observe these poles locating to RHP. How come this cannot be observed in matlab? While this is very likely because the model in Matlab is just a simplification, I do not understand how the bode plot can show a negative PM while the poles reside within LHP. Any thoughts?
Prashant Kumar answered . 2024-05-17 20:51:52
sys=zpk([2.4*10^9*j -2.4*10^9*j], [-0.0000001 -11.5*10^7+1.2*10^9*j -11.5*10^7-1.2*10^9*j],-100000000); subplot(121);bode(sys);grid; subplot(122);pzmap(sys); xlim([-1.5e+8 2e+7]);
The first subplot shows full bode plot. If you focus on the region you are looking at, you will see similar plot to what you show:
subplot(121); bode(sys, {1e+9*0.1034 1e+9*2.8278}); grid;
allmargin(sys)
isstable(sys)
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