A Note on Analysis of Diode Circuits

        Whenever you want to analyze a diode circuit, there are some simple rules that will help you figure out what happens in the circuit.  We will use this symbol, and these definitions of variables in this discussion.

Further, we will assume that the voltage-current curve for the diode looks like the heavy line on the graph below.

• The simplest thing to do in a diode circuit is to start by imagining that the diodes are all ideal.

• If an ideal diode is conducting current, the diode current, I_d,  has to be positive and there will be no voltage across the diode.  The usual paraphrase for this is that the diode is replaced by a short.  In the circuit below, when the diode is conducting, the situation looks like the one at the right.  (Note that the input voltage would have to be positive.)

• If an ideal diode is not conducting current, the diode current, I_d,  has to be zero and the voltage across the diode will be negative.  The usual paraphrase for this is that the diode is replaced by an open.  In the circuit below, when the diode is not conducting, the situation looks like the one at the right.  (Note that the input voltage would have to be negative.)

If you need to account for the non-ideality of the diode, there is a step you can take in that direction without using the full exponentially nonlinear nature of the diode.  That step is to use a model that has a voltage-current curve that looks has a known constant voltage when the diode is conducting.  A value of 0.8 volts is about right, and a circuit model looks like the one below.  The diode in this model is ideal, and the 0.8 volt voltage source accounts for most of the non-ideality.