# Feedback systems using passive systems

I started reading control system (book) just a few days ago , In the derivations of equivalent transfer function of Cascade or feedback connections it assumes that

"The subsystem connected to another subsytem doesn't load it"

1.Is that is reason why "only" passive systems are are not that important for designing control systems

1. with The use of only passive systems, can we design any feedback system ? Because I cannot visualize how can a passive system acts in a closed loop, is there any example of it?
• There is no consensus on this issue. For example, some believe that a current-limiting resistor connected in series to an LED introduces negative feedback. Or, there is a negative feedback in a charging capacitor. I personally think there needs to be amplification (active element, amplifier) to talk about a system with negative feedback. Aug 9 '20 at 11:59
• "In the derivations of equivalent transfer function of Cascade or feedback connections it assumes that..." If loading is present, it just means you cant use the ready made formulae. You can still derive equivalent transfer functions from first principles or regular circuit analysis techniques.
– AJN
Aug 9 '20 at 13:59

If your intention is TO DESIGN a control system for a given existing plant using only passive components, then it would be difficult to achieve your goal as you would lose at least two fundamental building blocks in control systems: (1) Subtractor ($$\e=r-y\$$), requiring an inversion in $$\y\$$) signal; (2) Amplifier with gain > 1. Also, that inversion in (1) is just an amplification with gain -1.