Circuits & Electronics
Circuits & Electronics ECE 3710
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Date Created: 11/02/15
ECE 3710 Diode Circuits Notes Ideal Diode An ideal diode is a diode that has zero current through it when it is reverse biased and zero voltage across it when forward biased In other words it is an opencircuit when reverse biased and a shortcircuit when forward biased The gure below shows the I V characteristics for the ideal diode 1D A iD Reverseblased Forwardbiased iD 0 OPeH CirCUit I D O shortcircuit Diode is off Diode is on VD Ideal V D 0 Piecewise Linear Approximation The piecewise linear model simpli es the analysis of circuits containing diodes and provides better approximation than the ideal diode model This model approximates the diode I V characteristics by straight line segments In each segment the approximation is linear so that linear circuit analysis can be used to analyze the circuit The gure below shows the circuit diagram of a piecewise linear model of a diode The circuit consists of an ideal diode a resistor and a DC voltage source ideal battery A Approximation slope D ideal V3 If Dlode curve ID IS 6 l VD RD E 5 VD 0 oi IDO For 12 lt VDO the diode is reverse biased off and I39D O For 12 gt VD0 the diode is forward biased on and the diode current is given by The gure above shows the graphs of the I Vplots for a typical silicon diode and for the piecewise linear approximation The breakpoint in the model occurs at 12 VDO To the left of this point the slope of the piecewise linear characteristic is zero To the right of 1 this point the slope is l R D By proper choice of the breakpoint and the slope a reasonable approximation to the diode I V characteristics can be obtained HalfWave Recti er Consider the circuit below We wish to nd the output voltage VL using the piecewise linear model for the diode D N VSt sinat RL VL First we replace the diode with its piecewise linear model as shown below r I Iideal RD VDO I 1L VSt sinat For VS gt VDO the ideal diode is forward biased on and it behaves like a shortcircuit Using KVL we can write V t V ha RDO D L On the other hand for VS lt VDO the ideal diode is reverse biased off and it behaves like an opencircuit The current iL t is apparently zero Using Ohm s law we can then write V t V RL for VStgt VDO VLtlLt39RL D L 0 for VStlt VDO The typical waveforms for VS t and VL t are depicted in the next gure This circuit is called a recti er because it passes current in only one direction It s a halfwave recti er because the current ows only during alternate half cycles of the applied voltage Peak Clipper Circuit The following circuit is used to clip off the peaks of a waveform We wish to nd the output voltage V0 as a function of the input voltage V In this diagram all the voltages are de ned with respect to ground This convention is assumed in most electronic circuit diagrams A detailed circuit diagram for the same circuit is shown below Note that all the ground nodes have the same potential and they should be considered as one node In other words all the grounds are connected This is very important as it helps us write down the loop KCL equations First we replace the diodes with their piecewise linear models as shown in the next gure V00 T VB RD l R ideal I W J V V1 VDO T 0 T RD V T B ideal For the top branch if V lt VDO VB the diode D1 is reverse biased off it behaves like an opencircuit Likewise for the bottom branch if VDO VB lt V the diode D2 is reverse biased off If both conditions hold the main middle branch is effectively cut off from the top and bottom branches and the circuit looks like the following l R r WW 0 For VD0VBltVIltVDOVB VI j VO Clearly i1 0 and V0 V For V gt VDO VB the top diode D1 becomes forward biased on Also the bottom diode D2 is still off so the all of the current 1391 ows to the top branch We can write a KVL equation from the ground node of V to the ground node of VB in the top branch V R1 l391RDl391VD0 VB 0 Rearranging we have 2 R1RD II To relate the output voltage V0 to the input voltage V we write another KVL equation from the ground node of V to the ground node of V0 V R1i1VO 0 Plugging in the expression for 1391 V0 V R1i1 VI VD0VB VR R1RD Rearranging we obtain an expression for V0 R For VI gtVD0VB V0 VD0VBI V139R R 1 D 1 D Note that the rst term is a constant and the second term is a linear function of the input voltage V Also note that R D is the internal resistor in the diode model and it s usually very small RD ltlt R1 Therefore the slope in this expression R1 RD is a small number In other words V0 is slowly increasing linear function of VI Similarly for V lt VD0 V5 D2 is on and D1 is still off In this case the top branch is disconnected to the circuit and the bottom branch is activated Using the same analysis we can show that R R ForVlt V VV V V V7D I D0 5 0 D0 5 R1RD I R1RD Plotting V0 as a function of V we get the following graph V0 Vim V3gt slope 1 R 1 D l S Ops R1RD 7VDU V1 R VDU Vs slope D R1R 7 VDU This circuit effectively limits the input voltage range from VDO V8 to VDO V3 The peak clipper circuit is often used at the input nodes of electronic devices as it prevents sudden spikes high voltage from damaging intemal circuitries The next gure demonstrates the inputoutput relationship of the peak clipper circuit V10 Va 1 VDo m 7 777777777777777777777 n V V30 m 7 r lquot t EIgt quot39 R V ED 17 4mm 7777777 V n l mom