2.6 Analyzing diode circuits

How to get started?

Here we adopt a different approach compared to other textbooks. We will not introduce you first to the most common diode circuits, and we will not explain how they work and make all relevant calculations. The idea is that you can make this analysis yourself after going through this section. It contains a lot of assessments that build up in complexity and that should allow you to gain more understanding in diode circuits.

Diodes - Level 1

OK, let’s start. In the first level, you have to estimate when the diode(s) will conduct in the given circuit. Use the second diode model, so a small knee voltage is needed to drive a current through the diode. Ignore the breakdown voltage, i.e. you may assume it is infinitely high.

Diodes - Level 2

In the next level, you will have to predict the influence of changing parameters in the circuit, e.g. what will happen if a certain resistor value increases. Again, use the second diode model. So, if the diode conducts, the voltage across the diode equals the knee voltage. If it does not conduct, the voltage is below the knee voltage.

Diodes - Level 3

Now we turn to numerical examples. The circuits in this level contain components with known values. Your task is to calculate the voltages and currents for these components. Assume a knee voltage of 1V.

Note: use a decimal point (not a comma) if your result contains a fractional part, e.g. V = 7.5 V.

Diodes - Level 4

Still some numerical challenges in this level, now with increased complexity. Only voltages are asked for now. Currents can be found by applying Ohm’s Law and KCL.

About mistakes

A little note on mistakes. Yes, mistakes are important. If you know that you made a mistake, you have learned something, and that’s what this course wants to help you with. In the following example, a mistake is made. Sometimes it is indeed not so obvious to predict whether or not a diode will conduct. You could start with an assumption, and then check whether the result makes sense.

Figure 2.16 Example of recognizing a mistake

What makes no sense?

  1. A negative current through the diode. A diode voltage below the knee voltage goes together with zero current.
  2. A diode voltage higher than the knee voltage. If there is a current, then the diode voltage equals the knee voltage.

Diodes - Level 5

Solve the next challenges. In some cases, you will have to make a mistake.

Node analysis

In some cases it might be needed to derive and solve an equation in order to find all voltages and currents. In this book, node analysis will be used mostly, as in the example below.

diode_node_analysis

Figure 2.17 Solving a diode circuit with node analysis

Here are the steps:

  1. Label the voltage at the upper node vo. Remember that this refers to the voltage at this node relative to ground.
  2. The voltage at the node to which the diode’s cathode is attached is then vo-1V, due to the 1V voltage drop across the diode.
  3. Estimate the direction of all currents entering and leaving the upper node. Note that it is not crucial to make correct estimates, since another direction will be reflected in your equation.
  4. Set up the equation by applying KCL in the upper node. In essence you apply Ohm’s Law to each resistor. Make sure you fill in the correct voltages.
  5. Now solve the equation for vo. (Up to you.)

Diodes - Level 6

You might need pen and paper now, to write down the equations…

Diodes - Level 7a

The goal of the next level is to find the relation between some input voltage and some output voltage. We will adopt a three-step approach. First, assume that the diode is on, and find a formula that expresses the output voltage as a function of the input voltage. Second, do the same thing, but now you assume that the diode is off. Third, combine both results. The input voltage at which both formula’s yield the same result is called the threshold voltage, and it is the voltage at which the diode starts or stops conducting.

Diodes - Level 7b

A bit harder…

Diodes - Level 7c

The last one of this section!

Go to 2.7 The Load Line

48 thoughts on “2.6 Analyzing diode circuits

  1. Great online course!
    Small remark, nothing major.. At diodes, level 3, question 2, the answer counts 7,5 as incorrect while the correct answer is 7.5 (notice the comma).

  2. It’s easier and more efficient to learn in this systematic and progressive way. Excellent tool! Can’t wait to see more chapters in this form! I think it will help us review past knowledge and prepare well for the exam, haha~

    • If the DC source voltage increases, the voltage at the diode’s cathode will also increase.
      As a result, the current through both resistors will increase, so also the current through the diode (obeying KCL).

          • Ok, I think I get it. Is it true that current through the diode is sum of resistors current only when voltagesource in the middle is 7V or higher?
            Thank you for your help!

          • Not necessarily. It depends on the value of both vin and the DC source, and these are not known.
            I think what you mean is that the current through the diode is the sum of resistors current only when the diode is on.

    • The voltage across the rightmost resistor is 0 V, because the diode voltage and the voltage across the rightmost voltage source cancel out. Applying KVL in the right loop (counterclockwise) yields: -1 V (source) + 1 V (diode) = 0 V (resistor).

    • Dear Joren.
      This might indeed be handy, but please help me on this. Can you suggest for which question(s) this might be useful?
      Hints are already included in most questions. If more or other hints are needed, tell me again for which questions.
      What do you mean with faults?

    • Well, the answer is correct though (c=6).
      I can’t tell where you made a mistake.
      Apply node analysis in the upper node, and label it (Vout + 1V).

  3. Can you maybe consider, when we click on an image to open it in a new tab.
    Because now when you click an image and you’re in the middle of a quiz, you have to do the quiz all over again, because you need to go back to the page. (reload it)

    • That would be quite difficult to achieve.
      I am using a plugin for the quiz, and I can’t change the code.
      Perhaps you could click with the mouse’s right button and indicate to open in a new tab?

  4. At question 7 of diodes: I think it would be a little easier to have a drawing of the circuit at the question of the graphic, cause I forget every time how my diode is situated

  5. Level 7c , question 7. Why is is the value of B equal -5 in place of 1?? I really don’t understand. Can you please explain, or see if I made a mistake? Thanks

        • Ah ok, well they do not represent some physical quantity. You could say that A/C is an amplification factor (that often is smaller than 1 though) and that B/C is a voltage.

          • then i do not understand how it works, is there a simple explanation somewhere for the calculation?

          • There’s no strict methodology to find A, B and C.
            The goal is to find the relation between vin and vout, and to put that relation in a specific format.
            E.g. if you find that vout = vin/2, than A would be 1, B = 0 and C = 2.

  6. Diodes – Level 5
    Question (2)
    Hello,
    I am having a difficult time to identify in which direction the current flows! and also what is the purpose of the 3v in the middle?
    Thank you!! 😀

    • If there is a current through the diode, it should be a forward current, so in the direction of the arrow in the symbol.
      There is not a specific purpose for the 3V source.

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