In my university EE lab we were working with a 1N4148 diode to rectify half a sine wave and measure the RMS value of the current in a simple circuit. To the left is the circuit we had to make and to the right it's the modeled circuit for theoretical calculations. However, from reading the datasheet (http://pdf.datasheetcatalog.com/datasheet/fairchild/1N4148.pdf) I find no reference not only to a model nor any mention of a resistance of 122 Ohm and the value of the DC voltage source of 0.48V. Is the model wrong, or am I just interpreting the datasheet wrong?
Typically datasheets concern themselves with worst-case values, and when you are doing modeling you at least want to start with nominal parameters. In the case of the Fairchild datasheet, no "typical" numbers are listed, so you're going to be looking at the graphs.
Figure 3 & 4 give you forward voltage vs. forward current, so the slope is dynamic resistance. The voltage at a given current can also be read from the graph, so subtract the voltage due to the bias current through the dynamic resistance at the operating point and you have the voltage source required.
Your voltage source + resistance model is inadequate to accurately describe the behavior with a large change in current however (it's okay as a small signal model linearized about an operating point). You can find a SPICE model for the 1N4148 and simulate the behavior or use the Shockley diode equation directly. The linked model shows a saturation current of 4.35nA and an ideality factor of 1.906.
The datasheets usually don't contain models for simple generic diodes. Some manufacturers may give out spice model for simulation purposes, but they are far more advanced than simply a resistor with a voltage source.
The model you happened to create is only a model, and it is specific to the environment it is in now (source voltage and impedance, load resistance, DC bias, AC signal). If that model is given to you or it is calculated from actual measurements then it should be correct.
The AC resistance is the ΔV/ΔI small signal changes that we call incremental resistance or Ri in Ω. This value is inversely proportion a=to all diode power ratings. e.g worst-case Ri=1/Pd 1Ω/W , best case 0.25/Pd
- 1 MOhm at 10 uA DC bias
- 50 Ohms at 1 mA DC bias
- 5 Ohms at 10mA DC bias
- 1 Ohm at 100mA DC bias
- 0.7 Ohms 800mA DC bias then flattens to 645 mOhms above this
Datasheets don't tell about the slope changes above saturation because the tolerances for VF at max current are too large and that is the load line slope above 100mA used as a switch.
But if you were modulating high current the change in voltage is actually less than the V/I load line slope. This incremental AC resistance, Ri or Rs or as effective AC series resistance or as I like to call it, ESR, like in cap ESR.
The log V vs log I is a straight line until mobility saturation is complete and the electrode semi bulk resistance then dominates the slope to an almost constant value.
Here is a set of 3 ON SEMI IV curves
Based on these parameters
I should mention high power currents must be <1% duty cycle to achieve these voltages otherwise the NTC Vf coefficient shifts the load line to a lower voltage with rising temperature and the PN junction Ri reduces slightly and speed improves.