Chapter #3: Diodes

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Chapter #3: Diodesfrom Microelectronic Circuits Textby Sedra and SmithOxford PublishingOxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)

Introduction IN THIS CHAPTER WE WILL LEARN the characteristics of the ideal diode and how to analyze anddesign circuits containing multiple ideal diodes together withresistors and dc sources to realize useful and interestingnonlinear function the details of the i-v characteristic of the junction diode (whichwas derived in Chapter 1) and how to use it to analyze diodecircuits operating in the various bias regions: forward, reverse,and breakdown a simple but effective model of the diode i-v characteristic inthe forward direction: the constant-voltage-drop modelOxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)

Introduction a powerful technique for the application and modeling of thediode (and in later chapters, transistors): dc-biasing the diode andmodeling its operation for small signals around the dc-operatingpoint by means of the small-signal model the use of a string of forward-biased diodes and of diodesoperating in the breakdown region (zener diodes), to provideconstant dc voltages (voltage regulators) application of the diode in the design of rectifier circuits, whichconvert ac voltages to dc as needed for powering electronicequipment a number of other practical and important applicationsOxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)

3.1.1. Current-VoltageCharacteristic of theIdeal Diode ideal diode – mostfundament nonlinearcircuit element two terminal device circuit symbol shownto right operates in two modes on and offOxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)Figure 3.1: Diode characteristics

3.1.1. Current-VoltageCharacteristic cathode – negative terminal, from which currentflows anode – positive terminal of diode, into whichcurrent flows voltage-current (VI) behavior is: piecewise linear for rated values nonlinear beyond this rangeOxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)

4.1.1: Current-VoltageCharacteristic of the IdealDiode mode #2: reversebias open ckt. ideal diode: is most fundamentdevice symbolnonlinearcircuit elementwithnodes twotwoterminaldevice with circuitsymbol to right operates in two modes forwardand reverse biasmode #1:forward bias short cktfigure 4.1.Oxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)

3.1.1. CurrentVoltageCharacteristic External circuit should bedesigned to limit current flow acrossconducting diode voltage across blockingdiode Examples are shown toright Oxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)Figure 3.2: The two modes ofoperation of ideal diodes and theuse of an external circuit to limit(a) the forward current and(b) the reverse voltage.

3.1.2: A SimpleApplication – TheRectifier One fundamentalapplication of thispiecewise linear behavioris the rectifier. Q: What is a rectifier? A: Circuit whichconverts AC waves into DC ideally with noloss.Oxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)Figure 3.3(a): Rectifier Circuit

3.1.2: A SimpleApplication – TheRectifier This circuit is composedof diode and seriesresistor. Q: How does this circuitoperate? A: The diode blocksreverse current flow,preventing negativevoltage across R.Oxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)Figure 3.3(a): Rectifier Circuit

3.1.3. AnotherApplication,Diode Logic Gates Q: How may diodes beused to create logic gates? A: Examples of AND /OR gates are shownright. Refer to next slide.Figure 3.5: Diode logic gates: (a)OR gate; (b) AND gate (in apositive-logic system).Oxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)

OR GATEAND GATEIF vA 5V THEN diodeA willconduct AND vY vA 5VIF vA 0V THEN diodeA willconduct AND vY vA 0VIF all diodes blockTHEN vY 5V 5VIF any diode conductsUniversity PublishingTHENvOxford MicroelectronicCircuitsbyAdelS. Sedra and Kenneth C. Smith (0195323033)Y 5V 5V-

3.2. TerminalCharacteristicsof Junction Diodes Most commonimplementation of adiode utilizes pn junction. I-V curve consists of threecharacteristic regions forward bias: v 0 reverse bias: v 0 breakdown: v 0Oxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)discontinuity caused bydifferences in scale

3.2.1. TheForward-Bias RegionIS constant for diode at giventemperature (aka. saturation current) The forward-biasregion of operation isentered when v 0. I-V relationship isclosely approximatedby equations to right.(3.3) is a simplificationsuitable for large vOxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)(Eq3.1) i IS (ev / VT 1)(eq4.1)VT thermal voltagek Boltzmann's constant (8.62E -5 eV/K)q magnitude of electron charge (1.6E -19 C)kT(eq4.2) 25.8mV(Eq3.2) VT q at roomtemperatureIS constant for diode at giventemperature (aka. saturation current)(Eq3.3) i IS ev / VT(eq4.3)

3.2.1. TheForward-BiasRegion Equation (3.3) may bereversed to yield (3.4). This relationshipapplies over as manyas seven decades ofcurrent.Oxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)IS constant for diode at giventemperature (aka. saturation current) i (eq4.4) v VT ln (Eq3.4) IS

3.2.1. TheForward-BiasRegion Q: What is the relativeeffect of current flow (i)on forward biasingvoltage (v)? A: Very small. 10x change in i, effects60mV change in v.step #1: consider two cases (#1 and #2)I1 IS eV1 / VT and I2 IS eV2 / VTstep #2: divide I2 by I1I2 IS eV2 / VT V1 / VTI1 IS estep #3: combine two exponentialsI2 e(V2 V1 ) / VTI1step #4: invert this expressionV2 V1 VT ln I2 / I1 step #5: convert to log base 10V2 V1 2.3VT log I2 / I1 Oxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)60 mV 2.3VT log 10 / 1

3.2.1: TheForward-BiasRegion cut-in voltage – is voltage,below which, minimalcurrent flows approximately 0.5V fully conducting region –is region in which Rdiode isapproximately equal 0 between 0.6 and 0.8VOxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)fully conducting region

3.2.2. The ReverseBias Regionthis expressionapplies fornegative voltages The reverse-bias region ofoperation is enteredwhen v 0. I-V relationship, fornegative voltages with v VT (25mV), is closelyapproximated byequations to right.Oxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)i IS e v / VTaction: invert exponential 1 i I S v / V e T 0 for largervoltagemagnitudesi IS

3.2.2. The ReverseBias Region A “real” diode exhibits reverse-bias current,although small, much larger than IS . 10-9 vs. 10-14Amps A large part of this reverse current is attributedto leakage effects.Oxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)

3.2.3. TheBreakdown Region The breakdown regionof operation isentered when v VZK. Zener-Knee Voltage(VZK) This is normally nondestructive.Oxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)breakdown region

i IS (ev / VT 1)i ISOxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)V 10VTi IS eV -VTV -VZKi IS v / VT

Oxford University Publishing Microelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033) Introduction a powerful technique for the application and modeling of the diode (and in later chapters, transistors): dc-biasing the diode and modeling its operation for small signals around the dc-operating

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