28-15 Calculate the dc beta (dc) for each of the following 28-26 What should an analog ohmmeter read

r read for both

values of dc: polarities of the meter leads when measuring across the
a. 0.9875. collector and emitter leads of a transistor?
b. 0.996. 28-27 Why do most DMMs have a special diode range for
c. 0.9975. checking diodes and transistor junctions?

SECTION 28–6 TRANSISTOR BIASING TECHNIQUES

SECTION 28–3 TRANSISTOR OPERATING REGIONS
28-28 What form of bias is shown in Fig. 28–25?
28-16 When a transistor operates in the active region, does
the collector current, IC, respond to changes in 28-29 In Fig. 28–25, solve for the following:
a. VCC? a. IB.
b. IB? b. IC.
28-17 In what region of operation does the collector of a c. VCE.
transistor act like a current source? d. IC(sat).
e. VCE(off).
28-18 In what region is a transistor operating if the collector
current is zero?

28-19 When a transistor is saturated, is IC controlled solely Figure 28–25

by IB?
VCC  12 V
28-20 How much is VCE when a transistor is saturated?

28-21 What is the ideal internal impedance of the collector

current source in Fig. 28–7b? RC  1.2 k
RB  220 k

SECTION 28–4 TRANSISTOR RATINGS

28-22 Calculate the power dissipation, Pd, in a transistor for ␤dc  100
each of the following values of VCE and IC:
a. VCE  5 V, IC  20 mA.
b. VCE  20 V, IC  50 mA.
c. VCE  24 V, IC  300 mA.
d. VCE  30 V, IC  600 mA.
28-30 Draw a dc load line for the transistor circuit in
28-23 A transistor has a power rating of 1.5 W at an ambient Fig. 28–25, and indicate the values of IC(sat), VCE(off), ICQ,
temperature, TA, of 25°C. If the derate factor is and VCEQ on the load line.
12 mW/°C, what is the transistor’s power rating at
each of the following temperatures? 28-31 In Fig. 28–25, recalculate the values of IB, IC, and VCE if
dc  150.
a. 50°C.
b. 75°C. 28-32 In Fig. 28–26, solve for the following:
c. 100°C. a. IB.
d. 125°C. b. IC.
e. 150°C. c. VCE.
d. IC(sat).
28-24 A transistor has a power rating of 2 W. Calculate the
maximum allowable collector current, IC(max), for each of e. VCE(off).
the following values of VCE:
a. 5 V.
Figure 28–26
b. 12 V.
c. 25 V. VCC  24 V

SECTION 28–5 CHECKING A TRANSISTOR WITH AN

OHMMETER RC  1.5 k
RB  680 k
28-25 When testing the BE and CB junctions of a silicon
transistor with an analog ohmmeter, what should the
meter show for both polarities of the meter leads if the
diode is ␤dc  200

a. good?
b. shorted?
c. open?

902 Chapter 28

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 28-33 Recalculate the values in Prob. 28–32 if RC  1 k . 28-38 In Fig. 28–29, solve for the following:
a. VB.
28-34 In Fig. 28–27, what value of RB will produce an ICQ of
3.75 mA and a VCEQ of 9 V? b. VE.
c. IC.
d. VC.
e. VCE.
Figure 28–27 f. IC(sat).
VCC  18 V
g. VCE(off).

RC  2.4 k
Figure 28–29
RB  ?
VCC  10 V

␤dc  150

RC  1 k
R1  8.2 k

␤dc  150

28-35 What form of bias is shown in Fig. 28–28?

28-36 In Fig. 28–28, solve for the following: R2  3.3 k RE  750 

a. VB.
b. VE.
c. IE.
d. IC.
e. VC. 28-39 For the pnp transistor in Fig. 28–30, solve for the
f. VCE. following:
g. IC(sat). a. VB.
h. VCE(off). b. VE.
c. IC.
d. VC.
e. VCE.
Figure 28–28
f. IC(sat).
VCC  24 V g. VCE(off).

RC  1 k Figure 28–30
R1  8.2 k
VCC  20 V

␤dc  100

RC  1.5 k
R1  6.8 k

R2  1.2 k RE  240 

␤dc  200

R2  2.2 k RE  1 k

28-37 Draw a dc load line for the transistor circuit in

Fig. 28–28, and indicate the values of IC(sat), VCE(off), ICQ,
and VCEQ on the load line.

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 28-40 What form of bias is shown in Fig. 28–31? 28-41 In Fig. 28–31, solve for IE and VC.

28-42 Recalculate the values in Fig. 28–31 if RC  1.5 k .

Figure 28–31
VCC  12 V

RC  1 k

␤dc  200

RB  1.5 k RE  1 k

VEE  6 V

Answers to Self-Reviews 28-1 a. the emitter region 28-4 a. 4.5 W

b. the collector region b. 600 mW
c. the base region
28-5 a. true
d. the emitter lead
b. false
28-2 a. forward/reverse
28-6 a. false
b. 120 A
b. increases
c. 0.992
c. 2.4 V
d. 180

28-3 a. the active region

b. the cutoff region
c. IB

Laboratory Application Assignment

In this lab application assignment you will examine two different supplied to you for this experiment. Keep each transistor
biasing techniques used with transistors: base bias and voltage separate.
divider bias. You will see that with base bias, IC and VCE are beta-
Q1, dc 
dependent values, whereas with voltage divider bias they are not.
Q2, dc 
Equipment: Obtain the following items from your instructor.
• Two 2N2222A npn transistors or equivalent Base Bias
• DMM Examine the circuit in Fig. 28–32. Calculate and record the
• Assortment of carbon-film resistors following dc values for each of the two transistor betas:
• Variable dc power supply
Q1 Q2
Beta, ␤dc
IB  IB 
Most handheld and bench-top DMMs available today are capable
of measuring the dc beta of a transistor. If your DMM has this IC  IC 
capability, measure and record the dc beta of each npn transistor VCE  VCE 

904 Chapter 28

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 Figure 28–32 Figure 28–33
VCC  15 V VCC  15 V

RB  390 k RC  1 k R1  12 k RC  1.5 k

VC

Q Q
VB
VCE

VE
 
VBE
R2  2.7 k RE  390 

Construct the circuit in Fig. 28–32. Measure and record the

following dc values for each of the two transistors:

Q1 Q2
Replace Q1 with Q2 and repeat the measurements listed.
IB  IB 
IC  IC  VB  VC 
VCE  VCE  VE  VCE 
IE
Voltage Divider Bias Which form of bias produces more stable results, base bias or
Examine the circuit in Fig. 28–33. Calculate and record the voltage divider bias? Explain your answer.
following circuit values:

VB  VC 
VE  VCE 
IE

Construct the circuit in Fig. 28–33. Measure the following circuit

values using Q1 as the transistor:

VB  VC 
VE  VCE 
IE

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