Task connected with diodes and linearization.

Discussion in 'Homework Help' started by Martyna, Jan 15, 2015.

  1. Martyna

    Thread Starter New Member

    Jan 15, 2015
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    I would be really grateful if somebody could show me how to linearize this circuit.

    Task:
    First linearize the circuit using through the use of the corresponding piecewise linear model of the diode, and then calculate the voltage at node ''X'' applying the principle of superposition.
    [picture attached]
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    It would be helpful to know which way the diode is facing.

    It would also be helpful so see YOUR best attempt to solve YOUR homework problem.

    What is the piecewise linear model of the diode you are using?
     
  3. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    Because of the markings around the diode (current Id, forward voltage Ufd) and the level of the two power sources i would say the diode is oriented with the cathode on the left and anode to the right.
    But i dont see any model for the diode either. If there is a certain piecewise linear model for the diode then we need to see that too. The linear model would make the diode appear to act as a resistor in series with another voltage source, and that would mean there is more series resistance than 2k plus 1k and a different voltage differential, so the current would be less than 1ma.
     
  4. Martyna

    Thread Starter New Member

    Jan 15, 2015
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    I am sorry for confusions. Probably I don't have enough informations provided to solve this task. That's why I didn't even know how to start it.
    Thank you all for answers.
     
  5. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hello,

    You are welcome, and if you like we can go through the analysis using a piecewise linear model from a typical diode like a 1N4148 for example.
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    Note that the task said to use "the corresponding piecewise linear model of the diode". That suggests that, somewhere in your material, a piecewise linear model for a diode is given that you are expected to use. This may be in the form of a graph. There are three common piecewise linear models:

    diode_models.png


    (A) is the ideal diode model.

    (B) is often called the ideal physical diode model. The voltage Vd is known as the diode forward voltage or often just the "knee" voltage.

    (C) is what most people think of when they hear "piecewise linear model" because the other two are just seldom ever called by that name except in introductory material. It is also sometimes called the large signal model. The value of Vd depends on the type of diode -- for instance, silicon diodes usually use Vd of 0.6V or 0.7V while germanium diodes usually use 0.3V.

    Does any of this look familiar to you?

    FYI: The fourth common model is the small signal model (or the incremental model). It consists of just the sloped part of (C) with the sloped line adjusted so that it is just tangent to the exponential diode I-V curve at the operating point. The reason that this model has no flat part below Vd is because it is defined and used only in a small region of operation around the operating point.
     
  7. Martyna

    Thread Starter New Member

    Jan 15, 2015
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    Thank you very much for your answer, now I will try to solve it.
     
  8. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    You can use the spice model to get two points on the diode curve and come up with the right values for the circuit. You can also make some measurements on a real life diode to get the two points you need. Once you approximate the current you have an idea where you want to linearize the response.
     
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