CHAPTER 10. Exercises. E10.1 Solving Equation 10.1 for the saturation current and substituting values, we have

Transcription

1 CHAPER 10 Exercises E10.1 Solving Equation 10.1 for the saturation current and substituting values, we have i Is exp( v / nv ) exp(0.600 / 0.026) A hen for v V, we have 15 i Is exp( v / nv ) exp(0.650 / 0.026) ma Similarly for v V, i ma. E10.2 he approximate form of the Shockley Equation is i I exp( v / nv ). aking the ratio of currents for two different voltages, we have i1 exp( v1/ nv ) exp ( v1 v2) / nv i exp( v / nv ) 2 2 Solving for the difference in the voltages, we have: v nv ln( i1/ i 2) hus to double the diode current we must increase the voltage by 0.026ln(2) mv and to increase the current by an order of v magnitude we need v ln(10) mv E10.3 he load line equation is V Ri + v. he load-line plots are shown on SS the next page. From the plots we find the following operating points: (a) V 1.1 V I 9 ma Q Q (b) V 1.2 V I 13.8 ma Q Q (c) V 0.91 V I 4.5 ma Q Q s 313

2 E10.4 Following the methods of Example 10.4 in the book, we determine that: (a) For R 1200 Ω, R 600 Ω, and V 12 V. (b) For R 400 Ω, R 300 Ω, and V 6 V. he corresponding load lines are: 314

3 At the intersections of the load lines with the diode characteristic we find (a) v v 9.4 V ; (b) v v 6.0 V. E10.5 Writing a KV equation for the loop consisting of the source, the resistor, and the load, we obtain: ( i i) v he corresponding load lines for the three specified values of i are shown: At the intersections of the load lines with the diode characteristic, we find (a) v v 10 V; (b) v v 10 V; (c) v v 5 V. Notice that o o the regulator is effective only for values of load current up to 50 ma. E10.6 Assuming that 1 and 2 are both off results in this equivalent circuit: o Because the diodes are assumed off, no current flows in any part of the circuit, and the voltages across the resistors are zero. Writing a KV equation around the left-hand loop we obtain v 1 10 V, which is not consistent with the assumption that 1 is off. 315

4 E10.7 Assuming that 1 and 2 are both on results in this equivalent circuit: Writing a KV equation around the outside loop, we find that the voltage across the 4-kΩ resistor is 7 V and then we use Ohm s law to find that 1 i equals 1.75 ma. he voltage across the 6-kΩ resistance is 3 V so i x is 0.5 ma. hen we have i 2 i i ma, which is not consistent with the assumption that 2 is on. x E10.8 (a) If we assume that 1 is off, no current flows, the voltage across the resistor is zero, and the voltage across the diode is 2 V, which is not consistent with the assumption. If we assume that the diode is on, 2 V appears across the resistor, and a current of 0.5 ma circulates clockwise which is consistent with the assumption that the diode is on. hus the diode is on. (b) If we assume that 2 is on, a current of 1.5 ma circulates counterclockwise in the circuit, which is not consistent with the assumption. On the other hand, if we assume that 2 is off we find that v where as usual we have referenced v 2 positive at the anode. 2 3 his is consistent with the assumption, so 2 is off. (c) It turns out that the correct assumption is that 3 is off and 4 is on. he equivalent circuit for this condition is: 316

5 For this circuit we find that i 4 5 ma and v 3 5 V. hese results are consistent with the assumptions. E10.9 (a) With R 10 kω, it turns out that the diode is operating on line segment C of Figure in the book. hen the equivalent circuit is: We can solve this circuit by using the node-voltage technique, treating v o as the node voltage-variable. Notice that v v. Writing a KC equation, we obtain vo 10 vo 6 vo Solving, we find v v V. Furthermore, we find that o i 1.39 ma. Since we have 6 V and i 0, the diode is in fact operating on line segment C. v (b) With R 1 kω, it turns out that the diode is operating on line segment B of Figure in the book, for which the diode equivalent is an open circuit. hen the equivalent circuit is: o Using the voltage division principle, we determine that v V. Because we have 6 v 0, the result is consistent with the assumption that the diode operates on segment B. 317

6 E10.10 he piecewise linear model consists of a voltage source and resistance in series for each segment. Refer to Figure in the book and notice that the x-axis intercept of the line segment is the value of the voltage source, and the reciprocal of the slope is the resistance. Now look at Figure 10.22a and notice that the intercept for segment A is zero and the reciprocal of the slope is (2 V)/(5 ma) 400 Ω. hus as shown in Figure 10.22b, the equivalent circuit for segment A consists of a 400-Ω resistance. Similarly for segment B, the x-axis intercept is +1.5 V and the reciprocal slope is (0.5 ma)/(5 V) 10 kω. For segment C, the intercept is -5.5 V and the reciprocal slope is 800 Ω. Notice that the polarity of the voltage source is reversed in the equivalent circuit because the intercept is negative. E10.11 Refer to Figure in the book. (a) he peak current occurs when the sine wave source attains its peak amplitude, then the voltage across the resistor is V V V and the peak current is 0.6 A. (b) Refer to Figure in the book. he diode changes state at the instants for which V sin( ω t) V. hus we need the roots of m B 20 sin( ω t ) 14. hese turn out to be ω t radians and ωt 2 π radians he interval that the diode is on is t 2 t ω 2π. hus the diode is on for 25.32% of the period. E10.12 As suggested in the Exercise statement, we design for a peak load voltage of 15.2 V. hen allowing for a forward drop of 0.7 V we require V 15.9 V. hen we use Equation to determine the capacitance m required. C ( I ) / V (0.1/60) / µ F. r m B E10.13 For the circuit of Figure 10.28, we need to allow for two diode drops. hus the peak input voltage required is V 15 + V / V. m r 318

7 Because this is a full-wave rectifier, the capacitance is given by Equation C ( I ) /(2 V ) (0.1/60) / µ F. r E10.14 Refer to Figure in the book. (a) For this circuit all of the diodes are off if 1. 8 < < 10. With the diodes off, no current flows and vo v on and 2 is in reverse breakdown. hen v V. When v in becomes less than -1.8 V diodes 3, 4, and 5 turn on and v V. he transfer characteristic is shown in Figure o 10.31c. (b) ) For this circuit both diodes are off if 5 < < 5. With the diodes off, no current flows and vo v in. in. o v o When v in exceeds 10 V, 1 turns v o When v in exceeds 5 V, 6 turns on and 7 is in reverse breakdown. hen v in 5 a current given by i (i is referenced clockwise) flows in the 2000 circuit, and the output voltage is i 0. 5v V vo in When v in is less than -5 V, 7 turns on and 6 is in reverse breakdown. vin + 5 hen a current given by i (still referenced clockwise) flows in 2000 the circuit, and the output voltage is vo i 0.5v 2.5 V in E10.15 Answers are shown in Figure 10.32c and d. Other correct answers exist. E10.16 Refer to Figure 10.34a in the book. (a) If v in () t 0, we have only a dc source in the circuit. In steady state, the capacitor acts as an open circuit. hen we see that 2 is forward conducting and 1 is in reverse breakdown. Allowing 0.6 V for the forward diode voltage the output voltage is -5 V. (b) If the output voltage begins to fall below -5 V, the diodes conduct large amounts of current and change the voltage v C across the capacitor. Once the capacitor voltage is changed so that the output cannot fall 319

8 below -5 V, the capacitor voltage remains constant. hus the output voltage is v vin v 2sin( ωt) 3 V. o C (c) If the 15-V source is replaced by a short circuit, the diodes do not conduct, v C 0, and v o v in. E10.17 One answer is shown in Figure Other correct answers exist. E10.18 One design is shown in Figure Other correct answers are possible. E10.19 Equation gives the dynamic resistance of a semiconductor diode as r nv / I. d Q I Q (ma) r d (Ω) , E10.20 For the Q-point analysis, refer to Figure in the book. Allowing for a forward diode drop of 0.6 V, the diode current is VC 0.6 IQ R C he dynamic resistance of the diode is nv rd I Q the resistance R p is given by Equation which is 1 Rp 1/ R + 1/ R + 1/ r C d and the voltage gain of the circuit is given by Equation Rp Av R + R p Evaluating we have V C (V) I Q (ma) r d (Ω) R p (Ω) A v

9 Problems P10.1 P10.2 P10.3 A one-way valve that allows fluid to flow in one direction but not in the other is an analogy for a diode. P10.4 he Shockley equation gives the diode current in terms of the applied voltage : v v i I s exp 1 nv where is the saturation current, and n is the emission coefficient I s which takes values between 1 and 2. he voltage V is the thermal voltage given by k V q 23 where is the temperature of the junction in kelvins, k joules/kelvin is Boltzmann's constant, and q coulombs is the magnitude of the electrical charge of an electron P10.5 k / q ( ) /( ) V emperature C Absolute temperature V (mv) i 321

10 P10.6* P

11 P10.8* he approximate form of the Shockley Equation is i I exp( v / nv ). aking the ratio of currents for two different voltages, we have i1 exp( v1/ nv ) exp ( v1 v2) / nv i exp( v / nv ) 2 2 Solving for n we obtain: v1 v n V ln( i1 / i2) 0.026ln(1/ 10) hen we have i 1 11 I s A exp( v1 / nv ) P10.9 At 175 K, we have: v (175 25) 0.35 V P10.10 For v 0.6 V, we have ( V ) i I v n s exp 1 I v nv s exp ( ) s hus, we determine that: 3 i Is exp v nv exp 0.6 ( ( ) ( ) A hen, for v 0.65 V, we have i I [exp(0.650/ 0.052) 1] ma s Similarly, for v V, we find i ma. P10.11 For part (a), Equation 10.3 gives the diode voltage in terms of the current as v nv ln ( i / Is ) + 1 For part (b) with a 20-Ω resistance in series, the terminal voltage is v v i A MAAB program to obtain the desired plots is: 323

12 log10_of_id -5:0.01:-2; id 10.^log10_of_id; vd 0.026*log((id/1e-14) + 1); v vd + 20*id; semilogy(v,id) hold semilogy(vd,id) (Note in MAAB log is the natural logarithm.) he resulting plots are shown: he series resistance is relatively insignificant for currents less than 0.1 ma and is certainly significant for currents greater than 10 ma P10.12 k / q ( ) /( ) 36.5 mv V i 1 5 I s A [exp( v1 / nv ) 1] i 2 Is [exp( v2 / nv ) 1] ma 324

13 P10.13* With the switch open, we have: i I ( v nv ) s exp 1 I s exp ( v nv ) hus, we determine that: Is A exp vv exp ( ) ( ) With the switch closed, we have: i1 i2 0.5 ma I exp v V s ( ) v nv ln 582 mv I s Repeating the calculations with n 2, we obtain: A I s v 564 mv P10.14 Using the approximate form of the Shockley Equation, we have 3 10 I s exp ( nv ) (1) 2 10 exp ( nv ) (2) I s ividing the respective sides of Equation (2) by those of Equation (1), we have exp ( ) 10 Is nv exp ( nv ) I exp nv ( ) s ( ) ln nv ( ) n V ln P10.15* (a) By symmetry, the current divides equally and we have I I 100 ma A B 325

14 (b) We have ( ) exp ( v nv ) i Is exp v nv 1 I Solving for I s, we obtain i Is vnv s exp ( ) For diode A, the temperature is 300 K, and we have V A I sa A 23 ka mv q A exp ( ) For diode B, we have 305 K, and 13 I 2I A sb sa V mv B Applying Kirchhoff's current law, we have 0.2 IA + IB ( 13 ) ( v ) ( 13 ) ( v ) exp exp Solving for v by trial and err, we obtain v mv, I 87 ma, and I B 113 ma. P10.16* he load-line equation is V Ri + v s s x x A Substituting values, this becomes 3 i + v x x Next, we plot the nonlinear device characteristic equation ix exp( vx) 1 /10 and the load line on the same set of axes. Finally, the solution is at the intersection of the load line and the characteristic as shown: 326

15 P10.17 he load-line equation is V Ri + v s s x x Substituting values, this becomes 20 5i + x v x in which i x is in millamperes and v x is in volts. Next, we plot the nonlinear device characteristic equation and the load line on the same set of axes. Finally, the solution is at the intersection of the load line and the characteristic as shown: 327

16 P10.18 he load line equation is Vs Ri s x + vx Substituting values this becomes 6 3 i + v x x Next we plot the nonlinear device characteristic equation i v 3 /8 the load line on the same set of axes. Finally the solution is at the intersection of the load line and the characteristic as shown: x x and P10.19 he load-line equation is Vs Ri s x + vx Substituting values this becomes i + v 3 x x Next we plot the nonlinear device characteristic equation i v v 2 x x + x and the load line on the same set of axes. Finally the solution is at the intersection of the load line and the characteristic as shown: 328

17 P10.20 (a) In a series circuit, the total voltage is the sum of the voltages across the individual devices. hus, we add the characteristics horizontally. he overall volt-ampere characteristic is (b) In a parallel circuit, the overall current is the sum of the currents through the individual devices. hus, we add the characterists vertically. he overall volt-ampere characteristic is 329

18 P10.21 he load-line construction is: At the intersection of the characteristic and the load line, we have v 1.85 V and i ma. P10.22 If we remove the diode, the hévenin equivalent for the remaining circuit consists of a 12-V source in series with a 3-kΩ resistance. he load line is At the intersection of the characteristic and the load line, we have the device current 1 i 3. 0 ma. hen, applying KC to the original circuit, we havei i 3. 0 ma

19 P10.23 If we remove the diode, the hévenin equivalent for the remaining circuit consists of a 5-V source in series with a 2-kΩ resistance. he load line is At the intersection of the characteristic and the load line, we have v 1.70 V and i 1. 7 ma. P10.24 If we remove the diode, the hévenin equivalent for the remaining circuit consists of a 10-V source in series with a 2.5-kΩ resistance. he load line is At the intersection of the characteristic and the load line, we have v 1.85 V and i ma. 331

20 P10.25 A Zener diode is a diode intended for operation in the reverse breakdown region. It is typically used to provide a source of constant voltage. he volt-ampere characteristic of an ideal 5.8-V Zener diode is: P10.26* he circuit diagram of a simple voltage regulator is: P10.27 Refer to Figure in the book. As the load resistance becomes smaller, the reverse current through the Zener diode becomes smaller in magnitude. he smallest load resistance for which the load voltage remains at 10 V corresponds to zero diode current. hen, the load current is equal to the current through the 100-Ω resistor given by i ( 15 10) / ma. hus, the minimum load resistance is R v / i 200 Ω. o P10.28 We need to choose R s so the minimum reverse current through the Zener diode is zero. Minimum current through the Zener occurs with minimum V s and maximum i. Also, we can write: Vs v iz i R Substituting values, we have 10 5 i Z R s s 332

21 Solving for the resistance we find R 50 Ω. Maximum power dissipation in the resistance occurs for maximum V s. 2 2 ( Vs max v ) (14 5) P max 1.62 W R R s s s P10.29 he diagram of a suitable regulator circuit is We must be careful to choose the value of R small enough so remains positive for all values of source voltage and load current. (Keep in mind that the Zener diode cannot supply power.) From the circuit, we can write Vs V Iz I R Minimum I occurs for I 100 ma and V 8 V. Solving for the z maximum value allowed for R, we have Vs V 8 5 Rmax 30 Ω I + I z hus, we must choose the value of R to be less than 30 Ω. We need to allow some margin for component tolerances and some design margin. However, we do not want to choose R too small because the current and power dissipation in the diode becomes larger as R becomes smaller. hus, a value of about 24 Ω would be suitable. (his is a standard value.) With this value of R, we have Vs max 5 IR max 208 ma R I max I max 208 ma Z R ( ) 2 PR max IR max R 1.04 W P 5I 1.04 W Z max Z max s I z P10.30 Refer to the solution to Problem P In the present case, we have R Ω, and we could choose R 8.2 Ω because this is a standard max

22 value and we need to provide some design margin. With this value, we have I I 610 ma R max Z max P R max P 3.05 W Z max P10.31 Refer to the solution to Problem P In the present case, we have R Ω, and we would choose R 2.4 Ω because this is a standard max 3.0 value and we need to provide some design margin. With this value, we have I I 2.08 A R max Z max P R max P 10.4 W Z max P10.32 First, find the hévenin equivalent for the circuit as seen looking back from the terminals of the nonlinear device: hen, write the KV equation for the equivalent circuit: Vt Ri t + v Next, plot this equation on the device characteristics and find the value of v and i at the intersection of the load line and the device characteristic. After the device current and voltage have been found, the original circuit can be solved by standard methods. P10.33* he hévenin resistance is R V / I 10 /2 5 Ω. Also the hévenin t oc sc voltage is V V 10 V. he load line equation is 10 5 i + v. We plot t oc the load line and nonlinear device characteristic and find the solution at the intersection as shown: ab ab 334

23 P10.34 An ideal diode acts as a short circuit as long as current flows in the forward direction. It acts as an open circuit provided that there is reverse voltage across it. he volt-ampere characteristic is shown in Figure in the text. After solving a circuit with ideal diodes, we must check to see that forward current flows in diodes assumed to be on, and we must check to see that reverse voltage appears across all diodes assumed to be off. P10.35 he equivalent circuit for two ideal diodes in series pointing in opposite directions is an open circuit because current cannot flow in the reverse direction for either diode. he equivalent circuit for two ideal diodes in parallel pointing in opposite directions is a short circuit because one of the diodes is forward conducting for either direction of current flow. 10 P10.36 (a) he diode is on, V 0 and I 3.70 ma (b) he diode is off, I 0 and V 10 V. (c) he diode is on, V 0 and I 0. (d) he diode is on, I 5 ma and V 5 V. 335

24 P10.37* (a) 1 is on and 2 is off. V 10 volts and I 0. (b) 1 is on and 2 is off. V 6 volts and I 6 ma. (c) Both 1 and 2 are on. V 30 volts and I 33.6 ma. P10.38 (a) is on, is on, and is off. V 7.5 volts and I (b) V in V I 0 on on on on on on on on 2 V 2 ma 6 off on on off 5 V 5 ma 10 off on on off 5 V 5 ma he plot of V versus V in is: P

25 P10.40 (a) he output is high if either or both of the inputs are high. If both inputs are low the ouput is low. his is an OR gate. (b) he output is high only if both inputs are high. his is an AN gate. P10.41 When the sinusiodal source is positive, 2 is on and 1 is off. hen, we have ( t ) 2.5 sin(2πt ). v o When the source is negative, 1 is on and 2 is off. hen, we have ( t ) 2.5 sin(2πt ). v o P10.42 If a nonlinear two-terminal device is modeled by the piecewise-linear approach, the equivalent circuit of the device for each linear segment consists of a voltage source in series with a resistance. P10.43 v R i + V a a 337

26 P10.44 We know that the line passes through the points (2 V, 5 ma) and (3 V, 15 ma). hus, the slope of the line is -1/R (-10 ma)/(1 V), and we have R 100 Ω. Furthermore, the intercept on the voltage axis is at v 1.5 V. hus, the equivalent circuit is P10.45 In the forward bias region (i > 0), the equivalent circuit is a short circuit. hus in the equivalent circuit, the voltage source is zero and the resistance is zero. In the reverse bias region (0 > v > -10 V) the equivalent circuit is an open circuit. hus in the equivalent circuit, the voltage is indeterminate and the resistance is infinite. In the reverse breakdown region (0 > i ), the equivalent circuit consists of a 10-V voltage source in series with zero resistance * he equivalent circuits for each segment are shown below: 338

27 For the circuit of Figure P10.46a, we can determine by trial and err (or by a load-line analysis) that the device operates on the middle line segment. hus, the equivalent circuit is: i 4.68 ma v i V For the circuit of Figure P10.46b, we can determine by trail and err that the device operates on the upper right-hand line segment. hus, the equivalent circuit is: i 10.4 ma v 800 i V P10.47* For small values of i, the Zener diode is operating on line segment C of Figure 10.19, and the equivalent circuit is 339

28 Writing a KC equation at node A, we obtain: v 13 v i Solving we obtain v i his equation is valid for v 6 V. When 0 6 V, the Zener diode operates on line segment B, for which the Zener is modeled as an open circuit and we have v i Plotting these equations results in v P10.48 (a) Assuming that the diode is an open-circuit, we can compute the node voltages using the voltage-division principle v V v V hen, the voltage across the diode is v v v 1 V. Because v is 1 2 greater than V 0. 7 V, the diode is not operating as an open circuit. f (b) Assuming that the diode operates as a voltage source, we can use KV to write: v v Placing a closed surface around the diode to form a super node and writing a KC equation gives v1 4 v1 v2 4 v Solving these equations, we find v V and v V. hen, writing a KC equation at node 1 gives the diode current. 4 v1 v i ma

29 Because the diode current is positive, the diode operation is consistent with the model. P10.49 Half-wave rectifier with a capacitance to smooth the output voltage: Full-wave circuits: P10.50 he peak value of the ac source is V V. hus the PIV is V and the peak current is ma. P10.51 he diode is on for V V sin( ωt ). Substituting values and solving, we B m find that during the first cycle after t 0 the diode is on for arcsin( 12/24) π / 6 ωt π arcsin(12/ 24) 5π / 6 he current is given by 24 sin( ωt ) 12 i ( t ) 12 sin( ωt ) 6 A 2 he charge passing through the circuit during the first cycle is 5π/6ω 5π/6ω 12 1 [12 sin( ωt ) 6] dt cos( ωt ) 6t ω π/6ω π/6ω Q m π ω he average current is the charge passing through the circuit in 1 second (or 60 cycles). Also, we have ω 120π hus π π 6 3 I avg A ω 120π π hen the time required to fully charge the battery is hours

30 P10.52 he dc output voltage is equal to the peak value of the ac source, which is v V. he load current is i v / R ma. he charge that passes through the load must also pass through the diode. he charge is Q i / mc. he peak inverse voltage is V. P10.53 (a) he integral of V m sin( ω t ) over one cycle is zero, so the dc voltmeter reads zero. (b) V avg 1 /2 V sin( ωt ) dt + 0 m /2 1 Vm 0dt cos( ωt ) ω t /2 t 0 V π m (c) V avg /2 1 2V Vm sin( ωt ) dt + Vm sin( ωt ) dt π 0 /2 P10.54* he output voltage waveform is: m he peak voltage is approximately 10 V. Assuming an ideal diode, the ac source must have a peak voltage of 10 V. he circuit is: he capacitance required is given by Equation in the text. I 0.1( 1 60 ) C 833 µ F V 2 r P10.55 As in Problem P10.54, the peak voltage must be 10 V. For a full-wave rectifier, the capacitance is given by Equation in the text: 342

31 ( ) ( ) I C 417 µ F 2V 2 2 r he circuit diagram is: P10.56 As in Problem P10.54, the peak voltage must be 10 V. For a full-wave rectifier, the capacitance is given by Equation in the text: ( ) ( ) I C 417 µ F 2V 2 2 r he circuit diagram is: P10.57 If we allow for a forward diode drop of 0.8 V, the peak ac voltage must be 10.8 V. Otherwise, the circuit is the same as in the solution to Problem P P10.58* For a half-wave rectifier, the capacitance required is given by Equation in the text. I 0.25 ( 1 60 ) C µ F V 0.2 r 343

32 For a full-wave rectifier, the capacitance is given by Equation in the text: I 0.25 ( 1 60 ) C µ F 2V r ( ) P10.59 (a) he current pulse starts and ends at the times for which v () t V s B 20 sin(200 π t ) 12 Solving we find that 1 sin (0.6) tstart ms and tend tstart ms 200π 2 Between these two times the current is 20 sin(200 πt ) 12 it () 80 A sketch of the current to scale versus time is (b) he charge flowing through the battery in one period is t end end 20 sin(200 πt ) 12 Q itdt () dt 80 t start t t start 1 12t cos(200 πt ) 800π 80 Q 194 µ C Finally, the average current is the charge divided by the period. 6 Q Iavg 19.4 ma P10.60 (a) With ideal diodes and a large smoothing capacitance, the load voltage equals the peak source voltage which is V m 12 V. hen the PIV is 2V m 24 V. t t end start 344

33 (b) Here again with ideal diodes and a large smoothing capacitance, the load voltage equals the peak source voltage which is V m 12 V. However the PIV is only V m 12 V. P10.61 (a) he circuit operates as three full-wave rectifiers with a common load and shared diodes. hus, the load voltage at any instant is equal to the largest source-voltage magnitude. he plot of the load voltage is is the period of the sinusoidal sources. (b) he minimum voltage occurs at t / 12 and is given by V V cosω /12 V cos( π / 6) V. hus the peak-to-peak ripple min m m is 0.134V m. he average load voltage is given by 1 Vavg v ( t ) dt 0 However, since v (t ) has 12 intervals with the same area, we can write: 1 /12 12 /12 Vavg v ( ) 0 t dt Vm cos( ω t ) dt / sin( π / 6) Vavg V m V m π (c) o produce an average charging current of 30 A, we require V V avg (d) In practice, we would need to allow for forward drops of the diodes, drops across the slip rings, and resistances of the stator windings and wiring. m 345

34 P10.62 A clipper circuit removes or clips part of the input waveform. An example circuit with waveforms is: We have assumed a forward drop of 0.6V for the diode. P10.63 Refer to Figure P10.63 in the book. When the source voltage is negative, diode 3 is on and the output v o (t) is zero. For source voltages between 0 and 10 V, none of the diodes conducts and v o (t) v s (t). Finally when the source voltage exceeds 10 V, 1 is on and 2 is in the breakdown region so the output voltage is 10 V. he waveform is: 346

35 P10.64 P10.65 P

36 P10.67 P10.68 A clamp circuit adds or subtracts a dc component to the input waveform such that either the positive peak or the negative peak is forced to assume a predetermined value. An example circuit that clamps the positive peak to +5 V is shown below: We have allowed a forward drop of 0.6 V for the diode. P10.69 his is a clamp circuit that clamps the positive peaks to zero. 348

37 P10.70* Refer to the circuit shown in Figure P10.70 in the book. If the output voltage attempts to become less than -5 V, the Zener diode breaks down and current flows, charging the capacitance. hus the negative peak is clamped to -5 V. he input and output waveforms are: P10.71 he capacitor and diode act as a clamp circuit that clamps the negative peak of v C1 1 A ( t) to zero. hus, the waveform at point A is: 349

38 iode 2 and capacitor C2 act as a half-wave peak rectifier. hus, the voltage across R is the peak value of ( ) v t V. his is v t. hus, ( ) 2 m called a voltage-doubler circuit because the load voltage is twice the peak value of the ac input. he peak inverse voltage is 2 for both diodes. A V m P10.72* A suitable circuit is: P10.73 A suitable circuit is: 350

39 P10.74 (a) A suitable circuit is: We choose the resistors R and 1 R2 1 R2 Slope 3 R + R 1 2 to achieve the desired slope. hus, choose R1 2R2. For example, R1 2 k Ω and R2 1 k Ω. (b) A suitable circuit is: Other resistor values will work, but we must make sure that remains forward biased for all values of v, including v 10 V. o achieve the desired slope (i.e., the slope is 0.5) for the transfer characteristic, we must have R R. 1 2 in in 2 351

40 P10.75* A suitable circuit is: We must choose the time constant RC >>, where is the period of the input waveform. P10.76 A suitable circuit is: We must choose R 1 to ensure that the 5.6-V Zener is in the breakdown region at all times and choose the time constant RC >>, where is the period of the input waveform. P10.77 I Q represents the dc component of the diode current with no signal applied to the circuit, and i d (t) represents the changes from the Q-point current when the signal is applied. Furthermore i (t) is the total diode current. hus we have i () t I + i () t cos(200 πt) Q d 2 352

41 P10.78 he small signal equivalent circuit of a diode is a resistance known as the dynamic resistance. he dynamic resistance is the reciprocal of the slope of the i versus v characteristic at the operating point. P10.79 c sources voltage sources are replaced by short circuits in a small-signal equivalent circuit. By definition the voltage across a dc voltage source is constant. hus, even if there is ac current flowing through the dc source the ac voltage across it is zero as is the case for a short circuit. P10.80 We should replace dc current sources by open circuits in a small-signal equivalent circuit. he current through a dc current source is constant. hus, the ac current must be zero even if we apply an ac voltage. Zero current for a non-zero applied voltage implies that we have an open circuit. P10.81* A plot of the device characteristic is: Clearly this device is not a diode because it conducts current in both directions. he dynamic resistance is given by: 1 di 8 r dv 3v 2 A plot of the dynamic resistance versus v is: 353

42 P10.82 We are given 6 10 i for 5 V < v 0 3 < 1+ v 5 ( ) A plot of this is: he dynamic resistance is: v di r dv 5 o find the dynamic resistance at a given Q-point, we evaluate this expression for v V Q. For I 1.0 ma, we have V 4.5 V and r 167 Ω. Q Q For I 10.0 ma, we have V 4.77 V and r 7.48 Ω. Q Q 354

43 P10.83 We are given v (t) cos(ωt) V and i (t) cos(ωt) ma. he dynamic resistance is the ratio of the ac voltage amplitude to the ac current amplitude. vd 0.01 r 50 Ω i d he Q-point results if we set the ac signals to zero. hus, we have V 5 V and I 3 ma Q Q P10.84 ynamic resistance is given by di 1 r dv dv di Because voltage is constant for changes in current, the dynamic resistance is zero for an ideal Zener diode in the breakdown region. P10.85* o find the Q-point, we ignore the ac ripple voltage and the circuit becomes: hus, we have: 8 5 I sq 150 ma 20 I ma Q I I I 100 ma Q sq Q he small-signal or ac equivalent circuit is: where r is the dynamic resistance of the Zener diode. Using the voltage-division principle, the ripple voltage across the load is Rp vac V ripple R + R p 355

44 1 where Rp 1 R + 1 r is the parallel combination of the load resistance and the dynamic resistance of the diode. Substituting values, we find R 3 p Vac R Solving, we find R p Ω. hen, we have: 1 Rp which yields r Ω. r p 356