Feedback systems using passive systems
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 &gt; 1. Also, that inversion in (1) is just an amplification with gain -1.
Opamps (the swiss knife for the analog designer) fill the requirements having (ideal) infinite open loop gain and input impedance.