• D1 ON, D2 OFF

• D1 ON, D2 ON

        To determine how this circuit works, you'll have to check every possibility.  We will start with the first case.  In this situation, we have:

• D1 OFF, D2 OFF

In this case, both diodes are OFF.

• Since both diodes are OFF, there is no current though either diode.  Consequently, there is no current through the resistor and V_out = 0.

• If V_out = 0, we have enough information to compute the voltage across each diode assuming that we know the input voltages.

• We can write KVL around either of two loops, and each loop will contain just one diode.

• Around the first loop we have:

• V_D1 = V₁ - V_out = V₁

• Since the voltage across the diode must be negative when there is no current through the diode we must have V₁ < 0.

• Around the second loop we have:

• V_D2 = V₂ - V_out = V₂

• Since the voltage across the diode must be negative when there is no current through the diode we must have V₂ < 0.

• We conclude:

• V_out = 0 when V₁ < 0 and V₂ < 0.

• In words, the output voltage is zero when both input voltages are negative.

        Now, consider the second case.  Here is the equivalent circuit for the second case

• D1 OFF, D2 ON

• Since D₂ is ON, it has been replaced by a short circuit, and that makes V_out = V₂.

• If D₂ is ON, the current must be positive, and that will occur only when V₂ > 0.

• If V_out= V₂, we have enough information to compute the voltage across D₁.

• We can write KVL around the loop that contains the resistor and D₁.  Around that loop we have:

• V_D1 = V₁ - V_out = V₁- V₂

• Since the voltage across a diode that is OFF must be negative, we have to have V₁< V₂.

• In words, when V₂ is positive and we have V₁< V₂, the output will be V₂.

        Now, examine the third case.

• D1 ON, D2 OFF

This case is exactly the same as the second case except that the two diodes are reversed.  The same argument we used for the second case works here with 1s and 2s interchanged, so we conclude:

• In words, when V₁ is positive and we have V₂< V₁, the output will be V₁.

        Finally, we get to the last case.

• D1 ON, D2 ON

• Since both diodes are ON, both diodes have been replaced by short circuits.

• The output voltage, V_out, is equal to both V₁ and V₂.

• The only way that can happen is if we have, V_out = V₁ = V₂.

• In words, when both input voltages are equal, that is what the output voltage becomes.

        We can summarize what happens in this circuit with a few simple statements.

• Given the diode circuit:\ below, and assuming that the diodes are ideal,

• When both input voltages are negative the output is zero.

• When either or both input voltages are positive, the output voltage is equal to the larger of the two input voltages.

What If I Want A Better Diode Model?

        We've been operating on the assumption that the diodes all act like our ideal model which has no voltage drop in the forward direction - when current flows.  The ideal model, and a theoretical voltage-current curve are shown below.

This is the model we've been working with.  A better - but still not exact model - is shown below.  You can see the model by clicking the small red button at the bottom right of the graph.

        This, new and improved - but not perfect - model can be modelled in terms of the first model we used - the ideal diode.  (It's not a perfect model of the diode because - as you can see - the two straight lines do not model the "corner" in the curve to perfection.)  A circuit model that gives the better voltage current curve is shown below - within the dotted lines around the circuit model.

The diode inside the model is ideal, in the sense that it has no forward drop across it when current flows through it.  The source in series with the ideal diode serves to account for the forward voltage drop - assumed constant in this model.  Note that the added voltage source serves to oppose the flow of curent until the voltage applied to the diode exceeds the threshold voltage, V,.  In the model above, the threshold voltage is 0.8v.

        There are still better models for diodes.  The diode has a nonlinear capacitance associated with it, for example.  You might want a more detailed model for the diode if you were using a simulation program and you wanted the results to be as exact as possible.  There are lots of other effects that could be modelled.  However, that's a topic for another lesson, another day.  That's it for this lesson.

        However, before you leave this lesson, be assured that the model we now have, and even the ideal diode model can often be used to predict performance of circuits with diodes, and they can help you understand those circuits.