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diode1 - ELEC 101 1 Diode Circuits 1 Diode(or PN junction...

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ELEC 101 Diode Circuits 1 2009/10 Spring 1 Diode (or PN junction diode) Two-terminals passive element with nonlinear I-V characteristics. First non-linear circuit element Symbol a Anode A Cathode K I D - V D Curve (or I D - V D Characteristics ) b V D > 0 0 Reverse breakdown region Reverse bias region Forward bias region I D V D < 0 I D V D p n Not in scale
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ELEC 101 Diode Circuits 2 2009/10 Spring Forward bias region 2 I V TH : threshold voltage (or called cut-in voltage , turn on voltage ) about 0.6V , voltage when diode just begins to conduct V TH V F V F : Diode voltage ; about 0.7 to 0.8V when a diode is in full conduction Reverse bias region 3 0 I S -V ZO I S : reverse saturation current very small and < 1 A V ZO : breakdown voltage , diode voltage when diode just begins to breakdown V TH , V F , I S , V ZO are diode dependent Diode is non-linear element and linear diode models are used to simplify diode circuit analysis . I D V D I D V D I D V D Can assume V F V TH ( = diode voltage drop) in some circuits V D
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ELEC 101 Diode Circuits 3 2009/10 Spring Diode models 4 Ideal diode (or switch) model a When diode is forward bias ( V D 0 ) , diode is like a closed switch (or called diode is ON ) Example Find V out . Assume diode D is ideal. V in V out V in V out 20V 0 20V 0 20V 0 V in V out 0 -5V V out (ideal diode) 20V 0 V in V in V out V out V in (V) 20 0 -5 D V in = 20V, V D 0 D is ON, V out = Vin When input voltage is large (>> V F ) diode can be modeled by ideal diode model When diode is reverse bias ( V D < 0 ) , diode is like an open switch (or called diode is OFF ) V out (real diode) 19.3V 0 V D < 0 V D 0 V in = -5V, V D < 0 D is OFF, V out = 0
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ELEC 101 Diode Circuits 4 2009/10 Spring Example Find V 1 . Assume diode is ideal. Assume diode D is ON , replace D by short Apply KCL 10 11 10 5 12 1 1 1 V V V V V 9V V 4 35 V 1 But if V 1 = 9V, D is OFF (reverse bias). Hence assumption (diode is ON) is wrong Assume diode D is OFF , replace D by open 1 10 V 12V 15 8V Assumption is valid and diode is OFF ( Assume diode is NOT breakdown. Usually breakdown voltage > 4V) 1-3 1-3 4 Steps to analyze diode circuits : 1. Assume the mode (region) of diode. 2. Replace diode by the diode model. 3. DC/AC circuit analysis. 4. Check, end, OR repeat steps 1 to 3 9V 4 8V 11V 11V V D = -3V Diode Circuits 5 5 10 10 11V 12V V 1 D 5 10 10 11V 12V V 1 D 5 10 10 11V 12V V 1 D 5 10 10 11V 12V D
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ELEC 101 Diode Circuits 5 2009/10 Spring Offset and Piecewise Linear diode models 6 Ideal diode (or switch) model a V F Offset diode model b I D V D I D V D V F Piecewise linear (PL) diode model c r D V F I D V D V F r D Ideal Diode Ideal Diode Diode Resistance Diode Voltage Diode Voltage V D > 0 0 V D < 0 I D V TH V F V D 0 V D < 0 I D V D < V F Ideal Diode V D V F I D V D V F I D V D < V F 0 0 0
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ELEC 101 Diode Circuits 6 2009/10 Spring Example Diode clipping circuit 7 A diode circuit to clip (cut) or reshape the input waveform.
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