Lecture 16Notes on Use of ModelsExample 1: Ideal Diode ModelGuessing the Diode Mode: GraphingSlide 5Guessing the Diode Mode: “Common Sense”Guessing the Diode Mode: When in Doubt…Slide 8Slide 9Example 2: Large-Signal Diode ModelExample 3: Large-Signal Diode ModelExample 4: Large-Signal Diode ModelExample 5: Ideal Diode ModelExample 6: Realistic Diode ModelLecture 16Today we willLearn how to solve diode circuitsBecome proficient in the application of the different diode I-V modelsGain experience “guessing” the correct diode mode for faster analysisNotes on Use of ModelsMost of the diode models are piecewise defined: One function for reverse bias Another for forward biasYou will need to: “Guess” that diode is reverse (or forward) biased Solve for V, I according to your guess If this results in an impossibility, guess againRarely, both guesses may lead to impossibility. Use a more detailed modelExample 1: Ideal Diode ModelFind ID and VD using the ideal diode model.Is the diode reverse biasedor forward biased?Make a guess, substitutecorresponding circuitfor diode.“Reality check”answer to see if we need to re-guess.+-IDVD+_2 V1 kVIReverse biasForward biasIV+_Guessing the Diode Mode: GraphingLook at the diode circuit as a Thevenin equivalent linear circuit attached to a diode.VL = VDIL = -IDGraph the diode I-V curve and the linear circuit I-V curve on the same graph, both in terms of ID and VD.This means draw the diode I-V curve normally, and draw the linear I-V curve flipped vertically (IL = -ID).See where the two intersect—this gives you ID and VD.Linear circuitIL+VL-IDVD+_Example 1: Ideal Diode ModelForward biasedVD = 0 VID = 2 mAVDID2 V2 mAGuessing the Diode Mode: “Common Sense”We may notice:Polarity of the 2 Vfalling over the resistor and diodeThe 2 V is in same direction as VDDiode is probably forward biased+-IDVD+_2 V1 kGuessing the Diode Mode: When in Doubt…It’s generally easier to guess reverse bias since it is easy to check.No matter what piecewise model we use, reverse bias is always open circuit.So when you don’t know what to do, put in open circuit for the diode, and see if it violates reverse bias conditions (zero current, negative voltage).Example 1: Ideal Diode ModelGuess reverse bias:Since no currentis flowing,VD = 2 V (by KVL)This is impossible for reverse bias (must have negative VD)So the diode must be forward biased+-IDVD+_2 V1 kExample 1: Ideal Diode ModelWe know diode isforward biased:VD = 0 VID = 2 V / 1 k = 2mASame as what we got graphically.+-IDVD+_2 V1 kExample 2: Large-Signal Diode ModelUse the large-signaldiode model with VF = 0.7 to find ID and VD.To be in forward biasmode, the diode needs0.7 V. The source only provides 0.5 V. The resistor cannot add to the voltage since the diode could only allow current to flow clockwise. Reverse bias => open circuit => ID = 0 A, VD = 0.5 V+-IDVD+_0.5 V1 kExample 3: Large-Signal Diode ModelUse the large-signaldiode model with VF = 0.7 to find ID and VD.+-ID2VD2+_2 VVD1+ -ID1Example 4: Large-Signal Diode ModelUse the large-signaldiode model with VF = 0.7 to find VX.+-Vx+_-2 V1 kExample 5: Ideal Diode ModelUse the idealdiode modelto find VX.+-Vx+_-2 VExample 6: Realistic Diode ModelUsing the realistic diode model with I0 = 10-6 A and VT = 0.026 V, compute ID and VOUT.+1
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