…I have to take something like 100kOhm for Ra (r2) if Rp (rp goes from 200Ohm to 600kOhm…

 r2 <- seq(5e3,1e6,20e3)
 rp <- seq(200,600000,5000)
 z <- outer(rp,r2,function(x,y){5/(x+y)*y})
 persp(rp,r2,z, 
      theta=55, 
      phi=20, 
      xlab="Rp",
      ylab="Ra", 
      zlab="U",
      ticktype="detailed",
      lwd=.1,
      col="black")