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  • Fig. 3-9 Simple zener diode reg. circuit & label zener terminals
  • Fig 3-2 b zener curve
  • Fig 3-3
  • Fig. 3-12 Zener Reg.
  • Fig. 3-12 Zener Reg.
  • Fig 3-16a,b,&c
  • Fig 3-20 depletion region
  • Fig 3-23
  • Fig 3-23
  • Fig 3-27
  • Fig 3-27
  • Fig 3-32
  • Fig 3-35
  • Fig 3-37 schem. current reg. diode
  • Fig 3-39 Schottky schem. & Fig 3-40.
  • Fig 3-41 schem symbol and diagram
  • Fig 3-47 a, b, & c
  • Fig 3-48b
  • Fig 3-49a&b zener reg
  • Fig 3-50 zener reg
  • EEE201 LECTURE 6

    1. 1. LECTURE-6 (EEE-201) Electronic Circuits-1 by Khondokar Fida Hasan
    2. 2. Special Diodes: Objectives  Describe the characteristics of a zener diode and analyze its operation  Explain how a zener is used in voltage regulation and limiting  Describe the varactor diode and its variable capacitance characteristics  Discuss the operation and characteristics of LEDs and photodiodes  Discuss the basic characteristics of the current regulator diode, the pin diode, the step-recovery diode, the tunnel diode, and the laser diode.
    3. 3. Introduction The basic function of zener diode is to maintain a specific voltage across its terminals within given limits of line or load change. Typically it is used for providing a stable reference voltage for use in power supplies and other equipment. RLoadThis particular zener circuit will work to maintain 10 V across the load.
    4. 4.
    5. 5. Zener Diodes – Operating RangeA zener diode is much like anormal diode, the exceptionbeing is that it is placed inthe circuit in reverse biasand operates in reversebreakdown. This typicalcharacteristic curveillustrates the operatingrange for a zener. Note thatits forward characteristicsare just like a normal diode. Operating range
    6. 6. Zener Diodes – Regulation RangesThe zener diode’s breakdown Zener zone Diode zonecharacteristics are determined bythe doping process. Low voltagezeners (>5V), operate in the zenerbreakdown range. Those designedto operate <5 V operate mostly inavalanche breakdown range.Zeners are available with voltagebreakdowns of 1.8 V to 200 V. Avalanche zone 5V. This curve illustrates the minimum and maximum ranges of current operation that the zener can effectively maintain its voltage.
    7. 7. Zener Diodes – Breakdown CharacteristicsNote very small reverse current (before “knee”).Breakdown occurs @ knee.Breakdown Characteristics:• VZ remains near constant• VZ provides: -Reference voltage -Voltage regulation• IZ escalates rapidly• IZ MAX is achieved quickly• Exceeding IZ MAX is fatal
    8. 8. Zener Diodes – Voltage RegulationRegulation occurs between: VZK - knee voltage to VZM - Imax
    9. 9. Zener Diodes – Equivalent Circuit• Ideal Zener exhibits a constant voltage, regardless of current draw.• Ideal Zener exhibits no resistance characteristics.
    10. 10. Zener Diodes – Equivalent Circuit• Zener exhibits a near constant voltage, varied by current draw through the series resistance ZZ.• As Iz increases, Vz also increases.
    11. 11. Zener Diodes – Characteristic Curve• ∆Vz results from ∆Iz.• ∆Iz thru Zz produce this. See Ex. 3-2
    12. 12. Zener DiodesZener diodes have given characteristics such as;• Temperature coefficients – describes the % ∆Vz for ∆Temp (0C) ∆Vz = Vz x T0C x ∆T  %/oC See Ex.3-3 (∆Vz)• Power ratings – the zener incurs power dissipation based on Iz and Zz  P = I2Z Power derating factor specifies the reduced power rating for device operating temperatures in excess of the “rated maximum temperature”. PD(derated) = PD(max) – (mW/0C)∆T  mWThe data sheet provides this information. See Ex.3-4 (%/oC)
    13. 13. Zener Diode – Data Sheet• Power ratings• Temperature ratings• Vz nominal• Impedance• Power derating curves• Temperature coefficients• ∆Zz - Zener impedance
    14. 14. Zener Diode - Applications RegulationIn this simple illustration of zener regulation circuit, the zener diode will“adjust” its impedance based on varying input voltages. Zener current willincrease or decrease directly with voltage input changes. The zener current, Iz,will vary to maintain a constant Vz.Note: The zener has a finite range of current operation. VZener remains constant
    15. 15. Zener Diode - Applications RegulationIn this simple illustration of zener regulation circuit, the zener diode will“adjust” its impedance based on varying input voltages and loads (R L) to beable to maintain its designated zener voltage. Zener current will increase ordecrease directly with voltage input changes. The zener current will increaseor decrease inversely with varying loads. Again, the zener has a finite rangeof operation. VZener remains constant See Ex. 3-5 Acrobat Document
    16. 16. Zener LimitingZener diodes can used for limiting just as normal diodes. Recall inprevious chapter studies about limiters. The difference to consider fora zener limiter is its zener breakdown characteristics. See Ex.3-8
    17. 17. Analysis of Zener Diode as a voltage regulator When,
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    26. 26. Varactor DiodesA varactor diode is best explained as a variable capacitor. Think of thedepletion region as a variable dielectric. The diode is placed in reverse bias.The dielectric is “adjusted” by reverse bias voltage changes.
    27. 27. Varactor DiodesThe varactor diode can be useful in filter circuits as the adjustable component forresonance frequency selection. Varactor Bias adjust
    28. 28. Varactor Diodes Capacitance Tolerance Range - This is the equivalent of the value tolerance range of a resistor. ie: 1N5148 – Nominal value = 47pFd - Tolerance range is 42.3pFd to 51.7pFd Tuning Ratio (TR) or (Capacitance Ratio) - Refers to Rangeability (value @ Bias Vmax vs Bias Vmin) - Vmin is 4V bias (C4) (for the 1N5139, C4 = 6.8pFd) - Vmax is 60V bias (C60) (for the 1n5139, C60 = 2.3pFd) For the 1N5139, TR = 2.9 Quality Factor (Q) describes energy loss in the device. High Q desireable) Temperature Coefficient - ∆Capacitance vs ∆Temp See pg.127Acrobat Document See Ex.3-9
    29. 29. Resonant Band-pass Filter w/ Varactor Diode Parallel Resonant Tank Series Resonant 31.6V Tank Varactor VR = 2.85 to 28.7VVBIAS = 2.9V to 29VCVaractor = 17pF to 55pF Varactor BiasResonant Frequency Range:fr = 679kHz to 1.22MHz. See Related Problem pg.130
    30. 30. Optical DiodesThe light-emitting diode (LED) emits photons as visible light.Its purpose is for indication and other intelligible displays.Various impurities are added during the doping process tovary the color output.
    31. 31. Optical DiodesElectroluminescence, the process of emittingphotons from a parent material (substrate), isthe basis for LEDs.Colors result from the choice of substratematerial and the resulting wavelength;Todays LEDs (green,red, yellow) are based onindium gallium aluminum phosphideBlue uses silicon carbide or gallium nitrideIR (infrared) – GaAs (gallium arsenide) A strong +bias encouragesLED Biasing: 1.2V to 3.2V is typical. conduction-band electrons in the N-material to leap theNote: Some newer LED’s run at higher voltages junction and recombineand emit immense light energy. Applications: with available holes Traffic signals releasing light and heat. Outdoor video screens Runway markers
    32. 32. LED – Spectral CurvesNote the wavelengths of the variouscolors and infrared.Note lead designations to the right.
    33. 33. LED Datasheet – MLED81 Infrared LED
    34. 34. LED Datasheet – MLED81
    35. 35. Optical DiodesThe seven segment display is an example of LEDs use for display ofdecimal digits. Acrobat Document See “Light Emitting Diodes.pdf”
    36. 36. PhotodiodesUnlike LED’s, photodiodes receive light rather than produce light. Thephotodiode varies it’s current in response to the amount of light thatstrikes it. It is placed in the circuit in reverse bias. As with most diodes, nocurrent flows when in reverse bias, but when light strikes the exposedjunction through a tiny window, reverse current increases proportional tolight intensity (irradiance).Note: Photodiodes allexhibit a “reverseleakage current” whichappears as an inversevariable resistance.Irradiance causes thedevice to exhibit areduction in thevariable resistancecharacteristic.
    37. 37. PhotodiodesYou have this one inyour kit.
    38. 38. Photodiodes – MRD821
    39. 39. Photodiodes – MRD821
    40. 40. Other Diode TypesCurrent regulator diodes(constant current diodes)keep a constant currentvalue over a specifiedrange of forward biasvoltages ranging fromabout 1.5 V to 6 V. Operating RangeThis device exhibits veryhigh impedances.
    41. 41. Other Diode TypesThe Schottky diode’s (hot-carrier diodes) significant characteristic isits fast switching speed. This is useful for high frequencies and digitalapplications. It is not a typical diode in that it does not have a p-njunction. Instead, it consists of a lightly-doped n-material andheavily-doped (conduction-band electrons) metal bounded together.Response is very quick…high speed digital communications. Lightly doped Heavily doped (conduction- band electrons)
    42. 42. Other Diode TypesThe pin diode is also used in mostly microwave frequency applications. Itsvariable forward series resistance characteristic is used for attenuation,modulation, and switching. In reverse bias it exhibits a nearly constantcapacitance. Also used in attentuators. Current-controlled Fixed Capacitance Resistance
    43. 43. Other Diode TypesThe step-recovery diode is also used for fast switchingapplications. This is achieved by reduced doping near thejunction. The diode recovers very quickly, making it useful inhigh-frequency (VHF) applications.
    44. 44. Other Diode TypesThe tunnel diode exhibits negative resistance. It will actually conduct wellwith low forward bias. With further increases in bias it reaches thenegative resistance range where current will actually go down. This isachieved by heavily-doped p and n materials that create a very thindepletion region which permits electrons to “tunnel” thru the barrierregion.Tank circuits oscillate but “die out” due tothe internal resistance. A tunnel diode willprovide “negative resistance” thatovercomes the loses and maintains theoscillations. Germanium or Gallium
    45. 45. Tunnel DiodesTank circuits oscillate but “die out” due to the internal resistance. Atunnel diode will provide “negative resistance” that overcomes theloses and maintains the oscillations.
    46. 46. Other Diode TypesThe laser diode (light amplification by stimulated emission of radiation)produces a monochromatic (single color) “coherent” light. Laser diodesin conjunction with photodiodes are used to retrieve data from compactdiscs. Forward bias the diode and electrons move thru the junction, recombination occurs (as ordinary). Recombinations result in photon release, causing a chain reaction of releases and avalanching photons which form an intense laser beam.
    47. 47. TroubleshootingAlthough precise power supplies typically use IC type regulators, zenerdiodes can be used alone as a voltage regulator. As with alltroubleshooting techniques we must know what is normal.A properly functioning zener will work to maintain the output voltagewithin certain limits despite changes in load.
    48. 48. TroubleshootingWith an open zener diode, the full unregulated voltage will bepresent at the output without a load. In some cases with full orpartial loading an open zener could remain undetected.
    49. 49. TroubleshootingWith excessive zener impedance the voltage would be higher than normalbut less than the full unregulated output.
    50. 50. Summary The zener diode operates in reverse breakdown. A zener diode maintains a nearly constant voltage across its terminalsover a specified range of currents. Line regulation is the maintenance of a specific voltage with changinginput voltages. Load regulation is the maintenance of a specific voltage for differentloads. There are other diode types used for specific RF purposes such asvaractor diodes (variable capacitance), Schottky diodes (high speedswitching), and PIN diodes (microwave attenuation and switching).
    51. 51. Summary Light emitting diodes (LED) emit either infrared or visible light whenforward-biased. Photodiodes exhibit an increase in reverse current with lightintensity. The laser diode emits a monochromatic light