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Analysis of circuits containing diodes

Discussion in 'Electronic Basics' started by [email protected], Jan 24, 2006.

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  1. Guest

    Hi,

    For the past couple of weeks I've been working through a book called
    "Network Analysis" by M.E. Van Valkenburg. Specifically, I've learned
    how to use Mesh/Nodal Analysis and Laplace Transforms to analyze simple
    circuits. I'm pretty amazed at the power of these techniques (thank
    you, Mr. Heaviside), even though I still have to learn more about
    differential equations and other things.

    Anyway, the book doesn't cover semiconductor components, and I would
    like to know how you would perform Nodal/Mesh Analysis on circuits that
    contain diodes. Is it difficult to do this type of analysis? I'm
    hoping to apply these techniques to a voltage doubler circuit to help
    me understand how it works, but I can't really do that without knowing
    how to address diodes.

    Thank you very much for your help!
     
  2. David Harmon

    David Harmon Guest

    On 23 Jan 2006 16:41:19 -0800 in sci.electronics.basics,
    wrote,
    http://home.comcast.net/~stager21/Circuits.html

    Essentially you have to try twice. once using the equation for a
    forward biased diode, and again for a reverse biased diode.
    (At the risk of over-simplifying.)
     
  3. Noway2

    Noway2 Guest

    You need to determine what model of a diode will suit your purposes.
    The most basic diode model is a current "valve" that lets current flow
    only one way combined with a relatively fixed voltage drop. If this
    simplified model does not provide enough accuracy for you, then you
    will need to include the equations governing the behavior of the
    device. The problem here is that the equations are non linear, often
    times involving differentials, logrithms, and or exponentials, which
    makes a closed form solution messy and difficult at best. My
    suggestion to you would be that if you need this detailed of an answer
    to resort to a computer simulation.

    For starters, however, you could consult a book on microelectronic
    circuits that covers diodes, transistors, etc.
     
  4. Guest

    Guest Guest

    wrote:
    : Hi,

    : For the past couple of weeks I've been working through a book called
    : "Network Analysis" by M.E. Van Valkenburg. Specifically, I've learned
    : how to use Mesh/Nodal Analysis and Laplace Transforms to analyze simple
    : circuits. I'm pretty amazed at the power of these techniques (thank
    : you, Mr. Heaviside), even though I still have to learn more about
    : differential equations and other things.

    : Anyway, the book doesn't cover semiconductor components, and I would
    : like to know how you would perform Nodal/Mesh Analysis on circuits that
    : contain diodes. Is it difficult to do this type of analysis? I'm
    : hoping to apply these techniques to a voltage doubler circuit to help
    : me understand how it works, but I can't really do that without knowing
    : how to address diodes.

    : Thank you very much for your help!

    Nodal Analysis is a technique that is applied to solve linear
    circuits.

    Diodes, BJTs, MOSFETs are non-linear elements, with multiple
    regions of operation. In order to solve circuits containing these
    elements, they must be replaced with linear models. The problem is that
    they use a different linear model for different regions of operation.

    A diode (to answer your initial question) can be modeled in one of
    two regions of operation: Forward or reverse biased (let's ignore
    avalance breakdown now, for simplicity.) A forward-biased
    can be modeled as a voltage source with the value of the forward voltage
    drop of the diode. A reverse biased diode can be modeled as an open
    circuit.

    To solve the circuit, you have to guess at which of the two
    regions of operation the diode is operating in, replace the diode with the
    appropriate linear model, and then solve the circuit with nodal analysis.
    Once the circuit has been solved, you need to check to see whether your
    guess was correct.

    In the case of the diode, if you guessed forward-biased, but the
    current through the diode was negative (i.e. flowing from cathode to
    anode) when you solved the circuit, you guessed wrong -- forward biased
    diodes necessarily must have positive current. Then you would re-solve
    the circuit using the reverse-biased model. Conversely, if you guessed
    reverse-biased, but found a voltage drop across the diode (modeled as an
    open circuit) greater than the forward voltage drop of the diode, you
    guessed wrong, because a diode with a large voltage drop (in the correct
    direction) across it would be forward-biased. Therefore, you would
    re-solve the circuit using the forward-biased model.

    This process is identical for other non-linear elements, the only
    difference is that the linearized models for those elements are more
    complicated, and have more regions of operation.

    Joe
     
  5. Guest

    Hi guys (David, Noway2, Joe),

    Rather than clutter up the newsgroup, I thought I would just write one
    thank-you to all of you.

    So if I understand what I've read, if the voltage source was, say, a
    square wave, the analysis could be even more complicated because the
    diode's bias could also be a function of time. That sounds pretty
    messy. Way over my head at this point. Simulation sounds like the
    best route for me.

    Thanks for your help!
     
  6. Guest

    Guest Guest

    wrote:
    : wrote:
    :> Hi,
    :>
    :> For the past couple of weeks I've been working through a book called
    :> "Network Analysis" by M.E. Van Valkenburg. Specifically, I've learned
    :> how to use Mesh/Nodal Analysis and Laplace Transforms to analyze simple
    :> circuits. I'm pretty amazed at the power of these techniques (thank
    :> you, Mr. Heaviside), even though I still have to learn more about
    :> differential equations and other things.
    :>
    :> Anyway, the book doesn't cover semiconductor components, and I would
    :> like to know how you would perform Nodal/Mesh Analysis on circuits that
    :> contain diodes. Is it difficult to do this type of analysis? I'm
    :> hoping to apply these techniques to a voltage doubler circuit to help
    :> me understand how it works, but I can't really do that without knowing
    :> how to address diodes.
    :>
    :> Thank you very much for your help!

    : Hi guys (David, Noway2, Joe),

    : Rather than clutter up the newsgroup, I thought I would just write one
    : thank-you to all of you.

    : So if I understand what I've read, if the voltage source was, say, a
    : square wave, the analysis could be even more complicated because the
    : diode's bias could also be a function of time. That sounds pretty
    : messy. Way over my head at this point. Simulation sounds like the
    : best route for me.

    Yeah, for simple circuits, with simple input signals, it's
    possible (and I'd recommend) going through the hand calculations to gain a
    greater intuition of the circuit.

    One thing that you might not be aware of (this is more applicable
    for BJTs and MOSFETs, rather than diodes) is that there exists a
    small-signal model. If your voltage source has a large DC bias with a
    small AC signal, then a different linear model for the non-linear elements
    could be employed to allow the circuit to be solved using nodal analysis,
    etc. As long as the varying part of the signal is small, the small signal
    approximations hold. However, for a large-signal input, like a full-scale
    square wave, these approximations do not hold.

    Simulators don't make these approximations. They use full-blown
    nonlinear models for the nonlinear elements, (for a diode, Id =
    Is*exp(vd*k) ) and use iterative methods to find the solution to the
    circuit.

    Joe
     
  7. Noway2

    Noway2 Guest

    The small signal model, around a linearized region, can be applied to a
    diode as well. The analysis process is almost identical to that of a
    BJT. The difference is that the diode will always attenuate the signal
    where the transistor can amplify it.
     
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