Doubt about Nyquist diagram

Good morning, I'm a new Mathworks community member, I'd like to ask something about Nyquist diagram: I know that stability of a system with feedback control can be evaluated by doing the Nyquist diagram of its open loop transfer function. if L is the the closed loop function of the feedback controlled system and A the open loop one, what is the difference between Matlab instructions nyquist(A) and nyquist(L)? Thanks for your support!

[Raj]

I am not sure what is the question here. You are absolutely right when you say "stability of a system with feedback control can be evaluated by doing the Nyquist diagram of its open loop transfer function". nyquist(A) will come into picture here. nyquist(L) will give you the nyquist plot of your closed loop system whose stability depends on your controller design. Can you please rephrare your question again to clarify what you are looking for?

[MartyD]

Sure, Thank you for answering me. I decided to create a feedback PD system, introducing the system transfer function in open loop A and the transfer function in closed loop L, guessing that the actuator and the Sensor are ideal, so their transfer functions are G2=H=1. My teacher shows us some diagrams about the L functions, changing progressively the Kd gain: one of these is about stability of controlled PD system and is obtained by the command nyquist(L). Perhaps I remember that Nyquist is used with A that is the open loop function, so I ask myself: can the stability be evaluated using directly the nyquist(L) instead of nyquist(A) when you know The transfer function in closed loop or this command line has another meaning?