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Below shows three sources with open terminals:

schematic

simulate this circuit – Schematic created using CircuitLab

In Figure 1 if we want to calculate the output resistance, according to the linear circuit theory we short the voltage source V1 and look into the source and calculate the Thévenin equivalent. Hence for Figure 1 the output impedance is simply 100 Ohm.

What should be the way of thinking for the output impedance/resistance for Figure 2? If we know the current-voltage characteristic and the dynamic resistance of the diode D1 what does that indicate? Because if we short V2 and apply voltage to its terminals, because it is reverse biased the current will be close to zero and we will experience almost infinite resistance.

Does that mean in Figure 2 the output resistance is infinity? Or it is the dynamic resistance? How should be the way of thinking to derive the output impedance/resistance for the Figure 2 source?

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    \$\begingroup\$ There is no constant output resistance of these circuits. They will have non-linear behavior depending on what they connected to. In case of ideal diodes you can speak of only two different "resistances". \$\endgroup\$ – Eugene Sh. Jan 15 at 15:42
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The Resistance can be plotted as a function of the DC output voltage.

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    \$\begingroup\$ This makes very sense to me. \$\endgroup\$ – cm64 Jan 15 at 17:40
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If you know the v-i curve of the diode, you can take the slope of the curve at any particular point and call it the incremental resistance. This, for instance, is the norm when designing with zener diodes (if you want to do it right). It allows you to calculate the output voltage for small(ish) variations of load. Keep in mind that zeners tend to have a much more linear response than signal diodes when operated above their voltage "knee".

However, since the incremental resistance varies with voltage/current, for regular diodes it is only valid for a small zone centered around the measurement point. So, "output resistance" in the simplest sense is not a valid concept for figure 2. The output resistance will vary depending on what is connected to the circuit.

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