Analysis of circuits containing diodes

On 23 Jan 2006 16:41:19 -0800 in sci.electronics.basics, snipped-for-privacy@hotmail.com wrote,

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.)

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.

snipped-for-privacy@hotmail.com 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

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!

snipped-for-privacy@hotmail.com wrote: : snipped-for-privacy@hotmail.com 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

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.