The most important question of
Semi Conductor Physics, Junction Diode Characteristics in Electronic Devices and Circuits;
Explain static resistance (DC) and dynamic resistance (AC)?, is being answered here.
DC or Static Resistance:
The application of a dc voltage to a circuit containing a semiconductor diode will result in an operating point on the characteristic curve that will not change with time. The resistance of the diode at the operating point can be found simply by finding the Corresponding levels of V
D and I
D as shown in Fig. 1.25 and applying the following Equation:
The dc resistance levels at the knee and below will be greater than the resistance levels obtained for the vertical rise section of the characteristics. The resistance levels in the reverse-bias region will naturally be quite high. Since ohmmeters typically employ a relatively constant-current source, the resistance determined will be at a preset current level (typically, a few mill amperes).
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| Figure 1.25 Determining the dc resistance of a diode at a particular operating point. |
AC or Dynamic Resistance:
It is obvious from Eq. 1.5 that the dc resistance of a diode is independent of the shape of the characteristic in the region surrounding the point of interest. If a sinusoidal rather than dc input is applied, the situation will change completely. The varying input will move the instantaneous operating point up and down a region of the characteristics and thus defines a specific change in current and voltage as shown in Fig. 1.27. With no applied varying signal, the point of operation would be the Q-point appearing on Fig. 1.27 determined by the applied dc levels. The designation Q-point is derived from the word quiescent, which means "still or unvarying".
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| Figure 1.27 Defining the dynamic or ac resistance. |
A straight line drawn tangent to the curve through the Q-point as shown in Fig. 1.28 will define a particular change in voltage and current that can be used to determine the ac or dynamic resistance for this region of the diode characteristics. In equation form,
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| Figure 1.28 determining the ac resistance at a Q-point |
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