So, As I am studying I keeping finding confusions in my concept of Diodes. Sorry if the question is too noobish. So in plotting the Vout with respect to Iin, my manual says that using the constant voltage model, Vout will be undefined in Iin<0. I don't understand why. I believe calculating using the first branch(with diode D1 and resistor) gives us something for Vout, but D2 dictates that Vout should be equal to VDon(.8 v).Is it a paradox? I don't understand.