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L5 special purpose diodes

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• Instructor: Muhammad Bilal Fig 3-20 depletion region
• Instructor: Muhammad Bilal Fig 3-27
• Instructor: Muhammad Bilal Fig 3-32
• L5 special purpose diodes

1. 1. Instructor: Muhammad Bilal Zener DiodesZener Diodes The Zener is a diode operated in reverse bias at the Zener Voltage (Vz). • When Vi ≥ VZ – The Zener is on – Voltage across the Zener is VZ – Zener current: IZ = IR – IRL – The Zener Power: PZ = VZIZ • When Vi < VZ – The Zener is off – The Zener acts as an open circuit
2. 2. Instructor: Muhammad Bilal Breakdown Characteristics Figure shows the reverse portion of a zener diode’s characteristic curve. As the reverse voltage (VR) is increased, the reverse current (IR) remains extremely small up to the “knee” of the curve. The reverse current is also called the zener current, IZ. At this point, the breakdown effect begins; the internal zener resistance, also called zener impedance (ZZ), begins to decrease as reverse current increases rapidly.
3. 3. Instructor: Muhammad Bilal Zener Equivalent Circuit Figure (b) represents the practical model of a zener diode, where the zener impedance (ZZ) is included. Since the actual voltage curve is not ideally vertical, a change in zener current (ΔIZ) produces a small change in zener voltage (ΔVZ), as illustrated in Figure (c). Z Z Z I V Z ∆ ∆ =
4. 4. Instructor: Muhammad Bilal Varactor Diodes A varactor diode is best explained as a variable capacitor. Think of the depletion region a variable dielectric. The diode is placed in reverse bias. The dielectric is “adjusted” by bias changes.
5. 5. Instructor: Muhammad Bilal Varactor Diodes The varactor diode can be useful in filter circuits as the adjustable component.
6. 6. Instructor: Muhammad Bilal Optical Diodes The light-emitting diode (LED) emits photons as visible light. It’s purpose is for indication and other intelligible displays. Various impurities are added during the doping process to vary the color output.