Fundamentals of Electronics PDF

Fundamentals of Electronics Dr.shiref Aboelnour 02 Junction Diode Characteristics and Diode Circuits For positive values For negative values: Comparison of Ge, Si, and GaAs diodes EXAMPLE 1.2 Using the curves of following fig : a. Determine the voltage across each diode at a current o...

Fundamentals of Electronics Dr.shiref Aboelnour 02 Junction Diode Characteristics and Diode Circuits For positive values For negative values: Comparison of Ge, Si, and GaAs diodes EXAMPLE 1.2 Using the curves of following fig : a. Determine the voltage across each diode at a current of 1 mA. b. Repeat for a current of 4 mA. c. Repeat for a current of 30 mA. d. Determine the average value of the diode voltage for the range of currents listed above. e. How do the average values compare to the knee voltages listed in Table 1.3 ? Solution: a. 𝑉𝐷 (𝐺𝑒) = 0.2 𝑉, 𝑉𝐷 (𝑆𝑖) = 0.6 𝑉, 𝑉𝐷 (𝐺𝑎𝐴𝑠) = 1.1 𝑉 𝑏. 𝑉𝐷 (𝐺𝑒) = 0.3 𝑉, 𝑉𝐷 (𝑆𝑖) = 0.7 𝑉, 𝑉𝐷 (𝐺𝑎𝐴𝑠) = 1.2 𝑉 𝑐. 𝑉𝐷 𝐺𝑒 = 0.42 𝑉, 𝑉𝐷 𝑆𝑖 = 0.82 𝑉, 𝑉𝐷 (𝐺𝑎𝐴𝑠) = 1.33 V d. Ge: 𝑉𝑎𝑣 = (0.2 V + 0.3V + 0.42 V)/3 = 0.307 V Si: 𝑉𝑎𝑣 = (0.6 V + 0.7 V + 0.82 V)/3 = 0.707 V GaAs: 𝑉𝑎𝑣 = (1.1 V +1.2 V +1.33 V)/3= 1.21 V e. Very close correspondence. Ge: 0.307 V vs. 0.3, V, Si: 0.707 V vs. 0.7 V, GaAs: 1.21 V vs. 1.2 V. RESISTANCE LEVELS RESISTANCE LEVELS DC or Static AC or Dynamic Average AC Resistance Resistance Resistance As the operating point of a diode moves from one region to another, the resistance of the diode will also change due to the nonlinear shape of the characteristic curve. 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 corresp onding levels of 𝑉𝐷 and ID as shown in Fig. 1.23 and applying the following equation: AC or Dynamic Resistance If a sinusoidal voltage is applied, the varying input will move the instantaneous operating point will change as well. The dynamic resistance 𝑟𝑑 𝑖𝑠 𝑔𝑖𝑣𝑒𝑛 𝑏𝑦 Average AC Resistance If the input signal is sufficiently large to produce a broad swing. The resistance associated with the device for this region is called the average ac resistance. The average ac resistance is, by definition, the resistance determined by a straight line drawn between the two intersections established by the maximum and minimum values of input voltage. Example 𝑟𝑎𝑣 = 5 Ω defines a value that is conside red the average of the ac values from 2 mA to 17 mA RESISTANCE LEVELS RESISTANCE LEVELS DC or Static AC or Dynamic Average AC Resistance Resistance Resistance DIODE EQUIVALENT CIRCUITS An equivalent circuit is a combination of elements properly chosen to best repre sent the actual terminal characteristics of a device or system in a particular op erating region. The result is often a network that can be solved using traditional circuit analysis techniques. Piecewise-Linear Equivalent Circuit Simplified Diode model Equivalent Circuit Ideal Equivalent Circuit Piecewise-Linear Equivalent Circuit To approximate the characteristics of the diode by straight-line segments. 1. The sloping section is the average resistance 𝑟𝑎𝑣. 2. The ideal diode is included to establish that there is only one direction of conduction. 3. A battery 𝑉𝐾 opposing the conduction direction must appear in the equivalent circuit. Simplified Equivalent Circuit For most applications, the resistance 𝑟𝑎𝑣 is sufficiently small to be ignored in comparison to the other elements of the network. Removing 𝑟𝑎𝑣 from the equivalent circuit is the same as implying that the characteristics of the diode appear Ideal Equivalent Circuit Now that 𝑟𝑎𝑣 has been removed. 0.7-V level can be ignored THANK YOU