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Diode Circuits

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The left hand diagram shows reverse bias, with positive on the cathode and negative on the anode (via the lamp). No current flows. The other diagram shows forward bias, with positive on the anode and negative on the cathode. A current flows. Practical Aspects of pn Junction anode cathode Forward bias Reversed bias

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Polarization of the pn Junction Forward bias examples (1) (2) (3) (4)

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Polarization of the pn Junction Reversed bias examples (1) (2) (3) (4)

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PN P N 1. DMM = 0 2. DMM = Diode Ohms Check Diode Ohms Check: Checks preformed on Si diode, by reversing the leads on the Digital Voltage Mutimeter (DMM). DMM + -

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Diode Voltages A conducting diode has about 0.6 volts across if silicon, 0.3 volts if germanium. To forward bias a diode, the anode must be more positive than the cathode or LESS NEGATIVE. To reverse bias a diode, the anode must be less positive than the cathode or MORE NEGATIVE.

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A Diode Puzzle Which lamps are alight? Some may not be full brightness

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A Diode Puzzle Which lamps are alight? Some may not be full brightness

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Exercise - a Diode Puzzle

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Which lamps are alight? Some may not be full brightness Exercise - a Diode Puzzle

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Diode Characteristic A diode is a nonlinear device and typical linear circuit analysis methods do not apply! circuit DR Rp Ev V reading A reading

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Diode Characteristic for Small-Signal Diodes less than 1mA at 300K When the temperature is increasing the knee voltage V knee decreases by about 2mV/K V knee n ~ 1-2 V T ~ 26 mV

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Analysis of Diode Circuits Nodal analysis Mesh analysis Kirchhoffs voltage law Thevenin & Norton theorems V th /R Th V th Slope=-1/R Th Example 10.1

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Analysis of Diode Circuits Thevenin equivalent ioio VoVo vDvD iDiD KVL KCL Their characteristics intersect

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Analysis of Diode Circuits Nodal analysis Mesh analysis Kirchhoffs voltage law Thevenin & Norton theorems V th /R Th V th Slope=-1/R Th Example 10.1

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Load-Line Analysis Problem If the circuit shown below has Vss=2V and R=1k and a diode with ch-tic shown, find the diode voltage and current at the operating point Repeat for: Vss=10V and R=10k V DQ =0.68V and i DQ =0.93mA

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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".diodecurrentbreakdown voltage

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Zener Diode - Voltage Regulator (reverse biased) Kirchhoffs voltage law V ss + Ri D +v D =0 Problem Find the output voltage for Vss=15V and Vss=20V if R=1k and a Zener diode has the ch-tic shown below. Load Line analysis Reverse bias region Slope of the load is -1/R

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Load Line Analysis of Complex Circuits Thevenin Equivalent

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Problem Consider the Zener diode regulator shown in figure (a). Find the load voltage v L and the source current i S if Vss=24V, R=1.2k and RL=6k

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Problem Consider the Zener diode regulator shown in figure (a). Find the load voltage v L and the source current i S if Vss=24V, R=1.2k and RL=6k Exercise – find Thevenin equivalent

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Problem Consider the Zener diode regulator shown in figure (a). Find the load voltage v L and the source current i S if Vss=24V, R=1.2k and RL=6k V T =V ss *(R L /(R+R L ))=20V R T =(RR L )/(R+R L )=1k Thevenin equivalent

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V T + R T i D + V D = 0 Load line equation Finally i S =(V SS -V L )/R=11.67 mA (from circuit a) Exercise 10.4 & 10.5 i D =-10mA V L =-V D =10V

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Ideal diode Model Useful for circuits with more than one diode (1)Assume a state for each diode, either on or off -2 n 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.

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Problem Analyze the circuit shown below using the ideal diode model. Start by assuming the D1 is off and D2 is on. 7V Not consistent with the assumption that D2 if off -3V Exercise 10.6 & 10.7 & 10.8

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Problem Analyze the circuit shown below using the ideal diode model. Start by assuming the D1 is off and D2 is on. 7V -3V Not consistent with the assumption that D1 is off

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Problem Analyze the circuit shown below using the ideal diode model. Start by assuming the D1 is off and D2 is on. 7V -3V This is OK

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Piecewise Linear Diode Models More accurate that the ideal diode model and do not relies on nonlinear equation 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 = R a i + V a

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Problem Find circuit models for the Zener-diode volt-ampere ch-tic shown in figure below 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=10 Repeat for the reverse bias Open circuit approximation Exercise 10.7

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