Models of Diode (Ideal and Constant Voltage)

-

Models of Diodes


  1.    Why use different models for diode?

  • we have seen how the I-V characteristic of a PN Junction diode reveals exponential dependence of junction current on biasing voltage, given by
  • This implies the necessity to solve exponential equations for obtaining exact values of current and voltages in circuits having diodes, which actually can't be expressed in closed form.
  • So to simplify our life, we adopt standard approximations for behaviour of diode, called models for analysing various circuits.



  2.    The IDEAL Diode Model

  • This model assumes that as soon as the diode is forward biased, even if the biasing voltage is just tiny bit greater than 0 volts, the diode acts as a short circuit and can be modelled by a short circuiting wire.
  • On the other hand, as soon as its reverse biased, it starts behaving as an open circuit, even though the biasing voltage is just infinitesimally negative.
  • In essence, all this approximation says is that as soon as you see the p side of diode facing positive(relative to n side) voltage, replace it with a short circuit, and if negative voltage( relative to n side) faces the p side, replace it with an open circuit.


Ideal model for a diode.

  3.    Constant Voltage Model

  • As seen earlier in the exponential behaviour, the current through the junction attains a significant value only after the biasing voltage reaches a certain value called built in potential, which is around 700mV for a typical silicon diode.
  • So this model tries to incorporate this built in potential for attaining greater accuracy.
  • Here, as the diode is considered to be switching between its "ON & OFF"  states for the biasing voltage V = builtin potential. 
  • Also, when diode is considered to be in its "ON" state, a drop of builtin potential is accounted for by adding a battery of emf equal to that potential. The following figure makes it very clear:


Constant voltage model of diode. Note: a battery is also included in this model, unlike the ideal model.

These models reduce our work tremendously, which will be clear when we'll see some applications.


Next post will be on Rectifiers!

Comments

Post a Comment

Popular posts from this blog

Biasing of PN Junction Diodes

-
Image
Biasing of Diodes   1.    What is biasing?  Having Understood the structure and equilibrium of a PN Junction diode without any external potential difference applied, its time now to see what happens one the application of an external potential difference: We know diodes are Directional  in nature, i.e their polarity matters due to the simple fact that they are  not symmetric  due to opposite charges on either side. So right off the bat we can predict application of potential can be done in two ways-   1. From n side to p side, 2. From p side to n side. Now, when we apply positive potential to P-side of diode, and of course ground / lower voltage to N-side the resulting configuration is called Forward Bias. Similarly when the opposite is done, resulting configuration is called Reverse Bias.     2.    Forward bias explained  As shown, the external electric field due to the battery points from p side to n side . This field opposes  the direction of field in the d
Read more

Rectifiers (Half & bridge)

-
Image
Rectifiers(half & full wave) Moving on to applications of diodes, an interesting application is Rectifiers. They are devices which convert an AC signal to a DC one . They are primarily of two types- half wave rectifier and full wave rectifier . The names would make sense once we have a thorough look at them.   1.    The Half Wave Rectifier This rectifier uses a diode and a capacitor for doing its job. Half Wave rectifier with load R_L We have to analyse its operation in two parts - one for the positive cycle of AC signal, other one for the negative cycle of signal . During the positive cycle of signal, diode D1 is forward biased , hence it allows the current to flow with ease. Also notice the signal source is parallel to C1 and R_l, so potential drop across them also follow the input voltage . Capacitor also gets charged upto (Vin-Vd). Now as the signal volt starts to drop after its peak , diode D1 goes to reverse bias state( since its +ive side
Read more

Basics of Diodes (Structure & Operation)

-
Image
PN Junction Diodes    1.    What are they? For starters, all you need to know for getting started are: Semiconductors like Silicon & Germanium  have 4 valence electrons in their normal state. When either a relatively electron  rich or electron  deficient  elements like Boron(3 valence electrons) or Phosphorus(5 valence electrons) are mixed with pure semiconductors (this mixing is formally called Doping ), we obtain n-type  or p-type  semiconductor. n-type  being the one with higher electron density  due to presence of electron rich element. Similarly p- type being the one with electron deficiency.  The electron deficient spots are considered as a positive charge carrier("assumed") and are called holes . Now when a layer(or wafer) of p-type & n-type semiconductors each are joined together, a strange rearrangement of charges takes place along with development of electric field, this bilayered structure is called a  PN Junction Diode  (a PN junction
Read more