PIN-Diodes | Integrators/Differentiators

Hello once again, my dear friends from r/AskElectronics

I have 2 questions today.

1.) How exactly do you regulate a PIN-Diode as a variable resistor based on the current that flows from the Anode to the Kathode?

2.) Can anyone explain the formulas of the Integrator/Differentiator to me? (Especially what you calculate with (-1/RC) for the inverted integrator and (-R*C) for the inverted differentiator.

Thanks a ton in advance.