Electronic – diode equivalent circuit model AC analysis

accapacitordiodeshigh frequencymodeling

Forward Biased AC Diode Model

I've understood the small-signal analysis for the diode, like how is the value for rd derived. But here are somethings i don't understand:

  1. How is capacitance of the diode dependent on the frequency of source?
  2. Why is capacitor in parallel with the resistor? Why not is series?
  3. How is all this different for reverse biased? With operating in high and low frequency?

Best Answer

all answers to your 3 questions are the same:

That's up to the model you're studying here, not universally fixed.

We can't tell you how the capacitance depends on the frequency in your model, because we don't know much about that model. To be exact, we only know that this is some model you've found, it applies to a forward-biased diode with a small AC signal atop of the DC bias, and that's it.

I'd agree, for example, that it'd make sense to see the junction capacitance in series with a resistance; but not the differential resistance \$r_d\$; so, you'd need to add one resistor. Bam! We've changed the model. It's now a different model than the one you're studying. Is that good? Maybe, our new model is more exact in some respect. Maybe the added complexity makes it impossible to calculate or explain something in the literature you're readin.

So: Models model a physical device, just well enough for a specific purpose. The assumptions made and descriptions of the elements of the model come with the model. Nobody else but the text you're reading can thus answer the questions you're sking here.

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