With normal diode, it is easy to define small signal model by derivative id as a function of Vd and this is equivalent to a resistor. However, what is the small signal model of an ideal diode (for example, one with Vd always exactly 0V or 0.7V)?

 

What diode model says that Vd is always exactly one of two possible voltages?

 

What diode model says that Vd is always exactly one of two possible voltages?

From this page there are two ideal models for diode:
A mathematically idealized diode:

Ideal diode in series with voltage source:

 

But look at those figures. Both of them allow for voltages other than 0 V for the top one or Vt for the bottom one. For instance, both allow a voltage of -1.5 V.

I think you mean to say that an ideal diode model has a Vd = Vt when Id ≥ 0 and Id = 0 when Vd ≤ Vt.

In that case, you use the same definition. Because the model is piecewise linear, you need two resistances, one for each region.

In the conducting region, what is the change in voltage due to a change in current?

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?

 

what is the small signal model of an ideal diode (for example, one with Vd always exactly 0V or 0.7V)?

It is a piece wise function, or a minimum function,

Vfwd=min(Vfwd, 0.7v).

 

This function has a dynamic resistance of infinity when Vfwd is less than 0.7v, and 0 otherwise.

 

In that case, you use the same definition. Because the model is piecewise linear, you need two resistances, one for each region.

In the conducting region, what is the change in voltage due to a change in current?

I think I've understood it now.
Voltage remains constant regardless of current variation. So it is like a voltage source here and it acts as a resistor with zero resistance in small signal model.

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?

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.

This function has a dynamic resistance of infinity when Vfwd is less than 0.7v, and 0 otherwise.

Yes, I see now.

 

See