From this page there are two ideal models for diode:What diode model says that Vd is always exactly one of two possible voltages?
It is a piece wise function, or a minimum function,what is the small signal model of an ideal diode (for example, one with Vd always exactly 0V or 0.7V)?
I think I've understood it now.In the conducting region, what is the change in voltage due to a change in current?
In this region, voltage changes but current remains the same so it is kind of current source that acts as a resistor with infinite resistance.In the non-conducting region, what is the change in voltage due to a change in current? The answer is easier to see if you think of it as what is the reciprocal of the change in current due to a change in voltage?
Yes, I see now.This function has a dynamic resistance of infinity when Vfwd is less than 0.7v, and 0 otherwise.