1. Chapter 4. Diodes

2. Copyright  2004 by Oxford University Press, Inc. Diode Simple non-linear device 2 terminal device, uni- or bi-directional current flow Semiconductor Many types –pn-junction diode –Photo diode –Light emitting diode –Zener diode

3. Copyright  2004 by Oxford University Press, Inc. Ideal Diode Cut-offTurned-on

4. Copyright  2004 by Oxford University Press, Inc. Rectifier

5. Copyright  2004 by Oxford University Press, Inc. Diode Logic Gates “OR” gate v Y is high if any of v A, v B, and v C is high. Y = A + B + C “AND” gate v Y is high if all of v A, v B, and v C are high. Y = A B C

6. Copyright  2004 by Oxford University Press, Inc. Given circuit is on the left. Assume the diodes are on (right.)

7. Copyright  2004 by Oxford University Press, Inc. Junction Diodes Semiconductor junction diodes made of silicon. – Forward bias region: v > 0 – Reverse bias region: v < 0 – Breakdown region: v < - V ZK - V ZK Forward bias regionReverse bias regionBreakdown region

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9. pn Junction of Diode Silicon crystals can be deliberately made impure to create an electrically unstable region. This process is called the “doping.” For example, boron makes p-type where more holes are present. Phosphorus makes n-type where more electrons are present.

10. Copyright  2004 by Oxford University Press, Inc. Open Circuit pn Junction I D = I S The depletion region in the middle creates a barrier voltage. Diode must be biased more than the barrier voltage to start conducting. Diffusion current Drift current

11. Copyright  2004 by Oxford University Press, Inc. Reverse Biased pn Junction Let I < I S to prevent the breakdown. Injection of additional electrons increases the width of depletion region by combining with more holes and electrons drifted to the middle.→ Increase the barrier voltage.

12. Copyright  2004 by Oxford University Press, Inc. Forward Biased pn Junction I supplies holes to p-region, creating more drift current I S. Depletion region narrows. → Decrease the barrier voltage.

13. Copyright  2004 by Oxford University Press, Inc. Forward Bias Region

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15. Reverse Bias Region

16. Copyright  2004 by Oxford University Press, Inc. Breakdown Region

17. Copyright  2004 by Oxford University Press, Inc. Exponential Model Graphical MethodNumerical Method (Iteration)

18. Copyright  2004 by Oxford University Press, Inc. Piecewise-Linear Model

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20. Constant Voltage Drop Model Usually assume V D = 0.7 V. Ideally, we can also assume V D = 0 V.

21. Copyright  2004 by Oxford University Press, Inc. No voltage drop.

22. Copyright  2004 by Oxford University Press, Inc. Small Signal Model Find the DC operating point (i.e., V D and I D ) first using any model. Then, for the small signal, model the diode as a resistor ( whose value equals the inverse of the slop at the Q point.

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24. Voltage Regulator

25. Copyright  2004 by Oxford University Press, Inc. Zener Diodes V Z = V Z0 + r Z I Z

26. Copyright  2004 by Oxford University Press, Inc. Zener Diodes as Shunt Regulator Put a Zener diode in parallel with a load. Voltage on the load is limited by the Zener voltage, V Z.

27. Copyright  2004 by Oxford University Press, Inc. Rectifier Circuits transformer

28. Copyright  2004 by Oxford University Press, Inc. Half Wave Rectifier Approximation Diode model

29. Copyright  2004 by Oxford University Press, Inc. Full Wave Rectifier

30. Copyright  2004 by Oxford University Press, Inc. Bridge Rectifier

31. Copyright  2004 by Oxford University Press, Inc. Peak Rectifier

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34. Precision Half Wave Rectifier Diode in feedback loop of voltage follower. Diode turns on by the open loop gain. → 0.7 V turn-on threshold not necessary.

35. Copyright  2004 by Oxford University Press, Inc. Limiting Circuit For circuits with passive elements only, K ≤ 1.

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37. DC Restorer Initially, the capacitor is charged to a certain voltage with diode on. At this point v O = 0V. Now the diode goes off when v I swings +10V from -6V to 4V. Since there is no discharging path, the capacitor keeps the same voltage; i.e., v C remains constant. Thus, the output must also swing +10V making v O = 10V.

38. Copyright  2004 by Oxford University Press, Inc. DC Restorer with Load R

39. Copyright  2004 by Oxford University Press, Inc. Voltage Doubler Clamp Peak rectifier During a positive cycle of input, C 1 is charged to V p. When the input passes the maximum, D 1 cuts off and D 1 turns on. This makes C 2 charged to 2V p

40. Copyright  2004 by Oxford University Press, Inc. Special Types of Diodes Schottky-Barrier Diode: Forward drop of 0.3 – 0.5 V Varactor: Acts as a voltage variable capacitor Photodiode: Conducts under the light in the reverse direction Ligh Emitting Diode: Converts the forward currents to lights.