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HW4_Soln

HW4_Soln - M asnari/EscutiB ri cldeY ECE 200 Fall 2008...

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M as nari/ E s cutiB ri c ld eY ECE 200 - Fall 2008 - Homework 4 Basic DC Circuits with Diodesand Capacitors l. Il2 ptsl Find the minimum vohage V* needed to keep the diode 'on'. Assume the diode hasa turn-sn voltage of 1 V' In order to detemine the minimum voltage F"*,n required to keep the diode'on', we must determine the condition that must existas we change V* so that the diode goes from ,off to ,on'. To do that,we i\$sume that the diode is 'off which implies that the voltage at node A with respect to ground is Yn <lv and I n = 0 ' As z" increases it wilL eventually cause I/, to just barely reach 1trl at which point the diode is turned 'on', but the cufreRt .Irthroughthe diode will still be 0. So arthat instant we have: (l) KW,*,o*: -3V -(2KO)/1 -VA * 0 where Vn =V, =lll -3V -tV Thus.I, =d=-ZmA' (2) KCL,*.^: Ir-Ir-rn =0where In =o sothat Ir=Ir=-2m4. (3) KWr*,,"*: Il A-(8KO)lr-Vx^n=0 where Vo=lY and Iz=-2mA' Thus we have V x,ou =l F * (8 K A) (zmA) = 17 V ln summary, diode is 'ofP when Vx <17V Yx >-l7Y + I, ) 0. f,t 4 I, = 0 and diode is 'on' when

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2. [10 ptsl Consider the circuit shown bolow. The diodehasa turn-on voltage of I V- a) D*errrrine the stde ofthe diode (i.e' is it 'on'_or toff?)' Note: Youmust cleartyshow all your work including any neatly &awn equivalent circuits that you might use. 1kf,) vYv Rr I* rl r ,rl Rri v 3ko Lsv+ + 4V Assume that the diode is 'on' in which case our KVLs and KCL can be written as: (1) KWuih"F: 4Y-(lKQ)/"*(3tKO)Iz=0 (2) KWnrn,^*: (3K Q)Ir*Vr *l'sV =0 where V, =lV (3) KCL,*,At Ix-Ir-Io=A Wecanso1veEq.(2)togetI,=w=0.833mAsothatEq.(1)becomes , * a"*(3{Q) (0'!31 =l.smA. we can now solve Eq(3) to give t*----Ga.' I o = I " - I o =1.5 mA* Q.833 mA = 0.667 mA. Thus since .r, > 0, the current flowing through the diode is in the proper direction indicating that our assumption of the diode being 'on' is correct. b) Use the stde ofthe diode found in part (a) to find the crur€nt /* flowing throughRr. As determined in part (a), thecurrent flowing through lt, it ll;ffi7l
3. tfa ptsl Assuming that theturn-oil voltage forthe diodes inthe circuits shown below is 0.7 v: a. Determine if the diode is 'on' in each circuit. b. Find the currerrts Ir, Iz aild Io in each circuit' c. Find the vohage d node X in each circuit. 3v 5V :- Cireuit I Circuit 2 CIRCUIT 1 (a) Assumingthatthe diode is 'ofP the KVL and KCL equations become: (1) KW"u,o^*: 5Il-(3KO)Ir -[JffD/, *3V =O (2) KCL,*"r : Ir* Ir- Ip =0 * /t =12 sinceln =0 Solving Eq. 1 for d we have (41(0)4 =(5Ir +37)+ 4=ZmA so that the voltage Z" at

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