# Small signal model for ideal diodes

Discussion in 'General Electronics Chat' started by anhnha, Mar 3, 2016.

1. ### anhnha Thread Starter Active Member

Apr 19, 2012
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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)?

2. ### WBahn Moderator

Mar 31, 2012
18,087
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What diode model says that Vd is always exactly one of two possible voltages?

3. ### anhnha Thread Starter Active Member

Apr 19, 2012
776
48
A mathematically idealized diode:

Ideal diode in series with voltage source:

4. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
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?

5. ### dannyf Well-Known Member

Sep 13, 2015
2,196
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It is a piece wise function, or a minimum function,

Vfwd=min(Vfwd, 0.7v).

6. ### dannyf Well-Known Member

Sep 13, 2015
2,196
417
This function has a dynamic resistance of infinity when Vfwd is less than 0.7v, and 0 otherwise.

7. ### anhnha Thread Starter Active Member

Apr 19, 2012
776
48
In that case, you use the same definition. Because the model is piecewise linear, you need two resistances, one for each region.

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 this region, voltage changes but current remains the same so it is kind of current source that acts as a resistor with infinite resistance.

Yes, I see now.

May 20, 2015
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