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Unformatted text preview: 280 Chapter 4 Bipolarjunction Transistors __________________———————- PROBLEMS. Section 4.1: Basic Operation of the
npn Bipolar Junction Transistor 4.1. Draw the circuit symbol for an npn BIT. Label the Q‘
terminals and the currents. Choose reference directions that J—
agree with the true direction of the current for operation in the :- active region.
Figure P4." 4.2. To forward bias a pn junction, which side of the junction i
should be connected to the positive voltage? In the active 4.12. Repeat Problem 4.11 if Q1 has 1551 = 10fA = 10‘14 A17 region, which type of bias (forward or reverse) is applied to the and ﬂ; = 100, whereas Q2 has 1552 = 100 EA = 10”13 A, am}
‘ :32 z 100. emitter—base junction? To the collector—base junction? Luz TWO transistors Q1 and Q2 connected in parallel are
equivalent to a single transistor as indicated in Figure P413.
If the individual transistors have 1551 = 1552 = 10‘13 A and 4.3. Sketch the input characteristic curve for a typical small-
signal silicon npn BIT at room temperature. Sketch the output characteristic curves if B = 250.
51 = [32 = 100, ﬁnd I 53 and £3 for the equivalent transistor._ 4'4' A certain npn Silicon uanSiStor has v” = 0‘7 V for Assume the same tem erature for all transistors
i B = 0.1 mA at a temperature of 30°C. Sketch the input p ' characteristic to scale at 30°C. What is the approximate value
of v” for is = 0.1 mA at 180°C? (Use the rule of thumb that v” is reduced in magnitude by 2 mV per degree increase in
temperature.) Sketch the input characteristic to scale at 180°C. 4.5. A certain npn silicon transistor has [3 =
i3 = 0.1 mA. Sketch ic against vcg for egg ranging from 0 to
5 V. Repeat for [8 = 300. Ignore second-order effects. 4.6. Sketch the output characteristics of a BJT, illustrating the
Early voltage and collector breakdown.
4. . An npn tran51stor has [3 E 200 and i3 = 0.1mA. The Figure P4J3
collector-to-emitter breakdown voltage 15 20V, and the Early _ _ .
voltage is VA = 100 V. Sketch ic against U“; to scale for the l 4.14.] Find the values of [i for the transrstors of Figure P414.
voltage range from 0 to 25 V. 4‘3 +15 V +15 V +15 V 4.8. An npn transistor is operating with the base—emitter
junction forward biased and the base—collector junction reverse
biased. If ic = 9mA for is = 0.3mA,ﬁnd £5, a, and [3. 4.9. A transistor has )3 = 50. What is the value of a ? 4.10. Consider an npn transistor at room temperature that has
[55 = 10”13 A, ﬂ = 100, 05‘}; = 10V, and i5 = 1135, v35, i3, i5, and ()1. (Assume that VT = 26 mV at room cg. \ Consider the circuit shown in Figure P411. The transistors
1 and Q2 are identical, both having [Es = 10 fA = 10-14 A and ﬂ = 100. Find V135 and lag. Assume a temperature of
300 K for both transistors. (Hint: Both transistors are operating
in the active region. Because the transistors are identical and
have identical values of V35 , their collector currents are equal.) ———'——" 282 Chapter 4 Bipolarjunction Transistors operation in ‘ ' . 4.34. Find I and V in the circuits shown in Figure P434, Fur;
Repeat for the cutoff region. all transistors, assume that ﬂ = 100 and |V35| = 0.7 in both
the active and saturation regions. Repeat for ,3 = 300. 4.27. Repeat Problem 4.26, for a pnp transistor. 4.28. Determine the region of operation for a room-
temperature silicon npn transistor that has )3 = 100 if
(a) VCE = 10V and [B = ZOMA; (b) [C = 13 ‘
(c) VCE = 3V and V35 = 0.4V; (d) [C = 1mA and
[B =‘- 50 MA.
4.29. Determine the region of operation for a room-
V temperature silicon pnp transistor that has B = 100 if I
(a) VCE = —5v and V3,; = —0.3V; (b) IC = 10mA 390m i +10 V +10V dI =1mA; I =0.05 A dV =—-5V.
an 3 (c) 3 m an 55 22 kg Section 4.5: Large-Signal DC Analysis
of BJT Circuits 4.30. Brieﬂy discuss the procedure for performing a dc analysis
of a BJT circuit using the large—signal circuit models. 4.31. Draw the ﬁxed-base bias circuit. What is the principal 1
reason that this circuit is unsuitable for the mass production of ——> '15 V ampliﬁer circuits? 1 kg
4.32. Draw the four-resistor bias circuit for the BJT. Give the + Q1 1‘
“ i ‘ 15 Mo Q2 rule—of—thumb design guidelines for this circuit.
g; 4. i 4.33; Use the large-signal models for the transistors illustrated in igure 4.19 to ﬁnd 1c and ch for the circuits of Figure P4.33.
Assume that 13 = 100. Repeat for [3 = 300 and compare the (c) ' (d) results for both values.
Figure P4.34 +20 V +20 V +15 V +15 V
4.35. Consider the circuit displayed in Figure P435. A Q- 47 kg 68 k9 value for 10 between a minimum of 4 mA and a ma '
470 m 5 mA is required. Assume constant resistor values, and su 1 that f] ranges from 100 to 300. It is desire
largest possible value while meeting the other constraints. 1 M9 the values of R B and R E. The resistorsi
required to be nominal values. " -15V " +15V
' +15V +15 V +15 V
1 k9 1M9 RB VBEQ = 0.7 V +
“r a Figure P4.35 4.36. Consider the four—resistor bias network of Fist“?
with VCC = 15v, R1 = 100m,122 = 47m, Rc = IMQ 10““ Figure P4.33 ...
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