0195136047.pdf

PROBLEMS 385

VS VZ RZ = 0 VL = VZ RL

VZ/RL

Source resistance Regulator resistor

DC source Regulator Load

+ RS I R

−

+

+

−

−

Figure P7.2.20

VS

D 1

D 2

500 kΩ v 1

500 kΩ

+

−

+

−

v 2

+

−

Figure P7.2.23

7.2.24Consider the circuit of Figure P7.2.20 withVS=

94 V,VZ =12 V,R= 820 ,RL= 220 ,

RS=0, andRZ = 25 . Assume the reverse

saturation current of the zener diode to be zero.

(a) Find the load voltage, current, and power.

(b) Calculate the power dissipated inRand in the

diode.

7.2.25For the circuit of Example 7.2.7 let the direction

ofD 2 be reversed. Find thei–vcurve.

*7.2.26Consider the periodic pulsating dc voltage pro-
duced by a half-wave rectifier. Find the Fourier
series representation and the average dc value.
7.2.27For the half-wave rectifier of Figure 7.2.8(a), let
the diode characteristic be the one given in Figure
P7.2.14(b) instead of being an ideal one. ForVS=
2 V andRL= 500 , sketchvL(t).
7.2.28Consider the circuit of Figure 7.2.9(a) withVS=
10 V,ω= 2 π× 103 rad/s,C= 10 μF, andRL=
1000 . SketchvL(t)and find the minimum value
ofvL(t)at any time after steady-state operation
has been achieved.
7.2.29For the rectifier circuit of Figure 7.2.9(a), sketch
the load current forC= 50 μF,R=1k, and
vS(t)=165 sin 377tV.
7.2.30Consider the bridge rectifier shown in Figure
P7.2.30. Describe its action as a full-wave rectifier,
assuming the diodes to be ideal.

7.2.31Consider a simple limiter circuit using ideal

diodes, as shown in Figure P7.2.31. Analyze its

action to restrict the variation of voltage within

certain limits.

7.3.1A transistor has a base currentiB = 25 μA,

α= 0 .985, and negligibleICBO. Findβ,iE,and

iC.

7.3.2A particular BJT has a nominal value ofα0.99.

Calculate the nominalβ.Ifαcan easily change

±1%, compute the percentage changes that can

occur inβ.

*7.3.3A silicon BJT has an emitter current of 5 mA at 300

K when the BEJ is forward-biased byvBE= 0. 7

V. Find the reverse saturation current of the BEJ.

NeglectingICBO, calculateiC,β, andiBifα= 0 .99.

7.3.4The parameters of a BJT are given byα= 0 .98,

ICBO=90 nA, andiC= 7 .5 mA. Findβ,iB, and

iE.

7.3.5For a BJT withvBE= 0 .7V,ICBO=4 nA,

iE=1 mA, andiC= 0 .9 mA, evaluateα,iB,iSE,

andβ.

7.3.6Consider the circuit of Figure P7.3.6 in which the

silicon BJT hasβ=85 and other typical values

at room temperature.ICBOmay be neglected.

(a) ComputeiB,iC,andiE, and check whether the

transistor is in the active mode of operation.

(b) Check what happens ifβis reduced by 10%.

(c) Check what happens ifβis increased by 20%.