EE40Lecture 26Venkat AnantharamLoad Line Analysis MethodIdeal Diode Model of PN DiodeLarge-Signal Diode ModelDiode: Large Signal ModelHow to Analyze Circuits with DiodesDiode Logic: AND GateSlide 1EE40 Spring 2008 Venkat AnantharamEE40Lecture 26Venkat Anantharam4/04/08 Reading: Chap. 10: DiodesSlide 2EE40 Spring 2008 Venkat AnantharamLoad Line Analysis Method1. Graph the I-V relationships for the non-linear element and for the rest of the circuit2. The operating point of the circuit is found from the intersection of these two curves.IVThev+−+V–RThevIVThevVThev/RThevVThe I-V characteristic of all of the circuit except the non-linear element is called the load lineoperating pointSlide 3EE40 Spring 2008 Venkat AnantharamIdeal Diode Model of PN DiodeCircuit symbol I-V characteristicreverse biasforward biasID(A)• An ideal diode passes current only in one direction. •An ideal diode has the following properties:• when ID> 0, VD= 0• when VD< 0, ID= 0VD(V)ID+VD–+VD–Switch modelIDDiode behaves like a switch: • closed in forward bias mode • open in reverse bias modeSlide 4EE40 Spring 2008 Venkat AnantharamLarge-Signal Diode ModelCircuit symbol I-V characteristicreverse biasforward biasID(A)VD(V)ID+VD–+VD–Switch modelID+−VDonVDonFor a Si pn diode, VDon≅ 0.7 VRULE 1: When ID> 0, VD= VDonRULE 2: When VD< VDon, ID= 0Diode behaves like a voltage source in series with a switch: • closed in forward bias mode • open in reverse bias modeSlide 5EE40 Spring 2008 Venkat AnantharamDiode: Large Signal Model• Use piece-wise linear modelSlide 6EE40 Spring 2008 Venkat AnantharamHow to Analyze Circuits with DiodesA diode has only two states:• forward biased: ID> 0, VD= 0 V (or 0.7 V)• reverse biased: ID=0, VD< 0 V (or 0.7 V)Procedure:1. Guess the state(s) of the diode(s)2. Check to see if KCL and KVL are obeyed.3. If KCL and KVL are not obeyed, refine your guess4. Repeat steps 1-3 until KCL and KVL are obeyed.Example: vs(t)+−If vs(t) > 0 V, diode is forward biased (else KVL is disobeyed – try it)+vR(t)–If vs(t) < 0 V, diode is reverse biased (else KVL is disobeyed – try it)Slide 7EE40 Spring 2008 Venkat AnantharamDiode Logic: AND Gate• Diodes can be used to perform logic functions:AND gateoutput voltage is high only if both A and B are highInputs A and B vary between 0 Volts (“low”) and Vcc(“high”)Between what voltage levels does C vary when the diode requires a nonzero forward bias voltage to turn on?VOUTVIN0505Slope =1Shift 0.7V UpEOCVccRANDA
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