For a specific application, I need to determine minimum Vds(in other form min Rds) and need to apply it to inverting input of an opamp. I have a source comes with huge ripple like below:
Before anyone suggest it, no, I can not simply measure the Vds on steady state and put a needed resistor for measured Rds. Because, supply voltage also varies between 13 and 11V and so ripple. Besides, output impedance also varies. So I need to develop an analog design to first, determine the minimum Vds then try to keep that potential on the opamp's inverting gate.
You can see the concept in the schematic below:
All the components in the circuit must be analog so please avoid to suggest digitally controlling the voltage.