summery for chapter 2, Study notes of Calculus for Engineers

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ELECTRONIC

CIRCUITS

(ELTL 322)

Chapter 2 :

Special Purpose Diodes

Objectives: ❖ Describe the characteristics of a zener diode and analyze its operation. ❖ Apply a zener diode in voltage regulation. ❖ Describe the varactor diode characteristic and analyze its operation. ❖ Discuss the characteristics, operation, and applications of LEDs, and photodiodes.

The symbol for a zener diode is shown in Figure 1. A zener diode is a silicon pn junction device that is designed for operation in the reverse-breakdown region. The diode characteristic curve of zener diode is shown in Figure 2 Figure 1 Figure 2

Zener Breakdown Two types of reverse breakdown in a zener diode are avalanche and zener. The avalanche effect occurs in both rectifier and zener diodes at a sufficiently high reverse voltage. Zener diodes with breakdown voltages of less than approximately 5 V operate in zener breakdown. Those with breakdown voltages greater than approximately 5 V operate in avalanche breakdown. Zeners are commercially available with breakdown voltages from less than 1 V to more than 250 V with specified tolerances from 1 % to 20 %.

Zener Equivalent Circuits Figure 4 represents the ideal model of a zener diode in reverse breakdown and its ideal characteristic curve. Figure 4

Figure 6 Figure 5 represents the practical model of a zener diode. A change in zener current (ΔIZ) produces a small change in zener voltage (ΔVZ), as illustrated in Figure 5. Figure 5

Zz =

∆Vz

∆Iz

Solution 2.

When 50mA

= 13m ╳ 3.5 = 0.045 V

So.

VZG= 6.8 + 0.045 = 6.845 V

When 25mA

= 12m ╳3.5 = 0.042 V

So.

V

ZL

= 6.8 - 0.042 = 6.758 V

Zener Diode Applications

Zener Regulation with a Variable Input Voltage

As the input voltage varies (within limits), the zener diode maintains a nearly constant output voltage across its terminals. However, as VIN changes, IZ will change proportionally so that the limitations on the input voltage variation are set by the minimum and maximum current values (IZK and IZM) with which the zener can operate.

Solution 2. This mean the zener diode can maintain an approximate 10 output voltage when the input voltage varies from 10.055V to 32 V. 𝑉𝑖𝑛(max) ≥ 𝑉𝑖𝑛 ≥ 𝑉𝑖𝑛(min) To calculate Vin(min) VR = IZK ╳R = 0.25 m╳220 = 55 mV VIN(min) = VR + VZ = 55 m + 10 = 10.055 V

IZM =

PD(max) VZ

1 10 = 100 mA VR = IZM ╳R = 100 m╳220 = 22 V VIN(max) = 22 + 10 = 32 V To calculate Vin(max)

Assignment 2. Determine the minimum and the maximum input voltage required for regulation to be established in Figure 9. Assume an practical zener diode with VZ = 5. 1 V at IZ = 49 mA, IZK = 1 mA , Zz= 7 Ω at Iz and, PD(MAX) = 1 W. For simplicity, assume this value of ZZ over the range of current values. Figure 9

When the output terminals of the zener regulator are open (RL = ∞), the load current is zero and all of the current is through the zener. When a load resistor (RL) is connected, part of the total current is through the zener and part through RL. The total current through R remains essentially constant as long as the zener is regulating. As RL is decreased, the load current, IL, increases and IZ decreases. The zener diode continues to regulate the voltage until IZ reaches its minimum value, IZK. At this point the load current is maximum, and a full-load condition exists.

Example 2. Determine the minimum and the maximum load currents for which the zener diode in Figure 11 will maintain regulation. What is the minimum value of RL that can be used? VZ = 12 V, IZK = 1 mA, and IZM = 50 mA. Assume an ideal zener diode where ZZ = 0 V and VZ remains a constant 12 V over the range of current values, for simplicity. Figure 11

Therefore, if RL is less than 490 Ω, RL will draw more of the total current away from the zener and IZ will be reduced below IZK. This will cause the zener to lose regulation. Regulation is maintained for any value of RL between 490 Ω and infinity.

Assignment 2. 2

Find the minimum and maximum load currents for which the circuit in Figure 11 will maintain regulation. Determine the minimum value of RL that can be used. VZ = 3. 3 V (constant), IZK = 1 mA, and IZM = 150 mA. Assume an ideal zener.

Varactor Diodes The junction capacitance of diodes varies with the amount of reverse bias. Varactor diodes are specially designed to take advantage of this characteristic and are used as voltage-controlled capacitors rather than traditional diodes. These devices are commonly used in communication systems. Varactor diodes are also referred to as varicaps or tuning diodes. Varactor Diode Symbol