Lecture5 diode circuits (1)

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Lecture5 diode circuits (1)

  1. 1. Diode Circuits
  2. 2. Practical Aspects of pn Junction anodeReversed bias + + Forward bias - - cathodeThe left hand diagram shows reverse bias, with positive on thecathode and negative on the anode (via the lamp). No current flows.The other diagram shows forward bias, with positive on the anodeand negative on the cathode. A current flows.
  3. 3. Polarization of the pn Junction (1) (2) Forward bias examples (3) (4)
  4. 4. Polarization of the pn Junction (1) (2) Reversed bias examples (3) (4)
  5. 5. Diode Ohms Check: Check Checks preformed on Si diode, by reversing the leads on the Digital Voltage Mutimeter (DMM). + - 1. DMM = 0 Ω P N N P DMM 2. DMM = ∞ Ω
  6. 6. Diode Voltages To forward bias adiode, the anode mustbe more positive than the cathode or LESS NEGATIVE. To reverse bias adiode, the anode must be less positive thanthe cathode or MORE NEGATIVE. A conducting diode has about 0.6 volts across if silicon, 0.3 volts if germanium.
  7. 7. A Diode PuzzleWhich lamps are alight? Some may not be full brightness.+ +- -
  8. 8. A Diode PuzzleWhich lamps are alight? Some may not be full brightness.+ +- -
  9. 9. Exercise - a Diode PuzzleWhich lamps are alight? Some may not be full brightness.+ +- -
  10. 10. Exercise - a Diode PuzzleWhich lamps are alight? Some may not be full brightness.+ +- -
  11. 11. Diode Characteristiccircuit A reading Rp R DEv V readingA diode is a nonlinear device and typical linear circuit analysis methods do not apply!
  12. 12. Diode Characteristic for Small-Signal Diodes   vd   i D = I s exp  nV  − 1    T  kT VT = n ~ 1-2 q VT ~ 26 mV Vknee less than 1mA at 300KWhen the temperature is increasing the knee voltage Vkneedecreases by about 2mV/K
  13. 13. Analysis of Diode Circuits Nodal analysis Mesh analysis Kirchhoff’s voltage law Thevenin & Norton theoremsVss = Ri D + v D Vth/RTh Slope=-1/RTh Vth Example 10.1
  14. 14. Analysis of Diode Circuits + io + Thevenin Vo equivalent vD - iD -KVL Vo = vD Their characteristicsKCL io = iD intersect
  15. 15. Analysis of Diode Circuits Nodal analysis Mesh analysis Kirchhoff’s voltage law Thevenin & Norton theoremsVss = Ri D + v D Vth/RTh Slope=-1/RTh Vth Example 10.1
  16. 16. Load-Line AnalysisProblemIf the circuit shown below has Vss=2V and R=1kΩ and a diode with ch-ticshown, find the diode voltage and current at the operating point Vss = Ri D + v DRepeat for:Vss=10V and R=10kΩVDQ=0.68V and iDQ=0.93mA
  17. 17. Zener Diode - Voltage Regulator (reverse biased) A Zener diode is a type of diode that permits current not only in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage".
  18. 18. Zener Diode - Voltage Regulator (reverse biased) Problem Find the output voltage for Vss=15V and Vss=20V if R=1kΩ and a Zener diode has the ch-tic shown below. Load Line analysisKirchhoff’s voltage law Vss+ RiD+vD=0 Slope of the load is -1/R Reverse bias region
  19. 19. Load Line Analysis of Complex CircuitsThevenin Equivalent
  20. 20. ProblemConsider the Zener diode regulator shown in figure (a). Find the load voltagevL and the source current iS if Vss=24V, R=1.2kΩ and RL=6kΩ.
  21. 21. ProblemConsider the Zener diode regulator shown in figure (a). Find the load voltagevL and the source current iS if Vss=24V, R=1.2kΩ and RL=6kΩ.Exercise – find Thevenin equivalent
  22. 22. ProblemConsider the Zener diode regulator shown in figure (a). Find the load voltagevL and the source current iS if Vss=24V, R=1.2kΩ and RL=6kΩ. Thevenin equivalent VT=Vss*(RL/(R+RL))=20V RT=(RRL)/(R+RL)=1kΩ
  23. 23. Load line equation VT + RTiD + VD = 0 VL=-VD=10V iD=-10mAFinally iS=(VSS-VL)/R=11.67 mA (from circuit “a”)Exercise 10.4 & 10.5
  24. 24. Ideal diode ModelUseful for circuits with more than one diode (1) Assume a state for each diode, either “on” or “off” -2n combinations (2) Assume a short circuit for diode “on” and an open circuit for diode “off” (3) Check to see if the result is consistent with the assumed state for each diode (current must flow in the forward direction for diode “on” and the voltage across the diodes assumed to be “off” must be positive at the cathode – reverse bias) (4) If the results are consistent with the assumed states, the analysis is finished. Otherwise return to step (1) and choose a different combination of diode states.
  25. 25. ProblemAnalyze the circuit shown below using the ideal diode model. Start byassuming the D1 is off and D2 is on. 7V -3V Not consistent with the assumption that D2 if offExercise 10.6 & 10.7 & 10.8
  26. 26. ProblemAnalyze the circuit shown below using the ideal diode model. Start byassuming the D1 is off and D2 is on. 7V -3V Not consistent with the assumption that D1 is off
  27. 27. ProblemAnalyze the circuit shown below using the ideal diode model. Start byassuming the D1 is off and D2 is on. 7V -3V This is OK
  28. 28. Piecewise Linear Diode ModelsMore accurate that the ideal diode model and do not relies on nonlinearequation or graphical techniques. (1) Diode V-I ch-tic approximated by straight line segments (2) We model each section of the diode I-V ch-tic with R in series with a fixed voltage source v = Rai + Va
  29. 29. ProblemFind circuit models for the Zener-diode volt-ampere ch-tic shown in figurebelow using the piecewise-linear diode model. Draw a line Look for intercept (0.6V) & the reciprocal of the slope (1/R) (1.6V-0.6V)/100mA=10Ω Open circuit approximation Repeat for the reverse biasExercise 10.7

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