1. Transistor Circuit Design Bipolar Transistors Heathkit EB-6002 Part 2

2. BIPOLAR TRANSISTORS - Introduction In this section, you will examine the structure, input and output characteristics, and selected parameters for bipolar transistors. In addition, appropriate models will be introduced that are used to analyze and design transistor circuits.

3. BIPOLAR TRANSISTORS - Structure Bipolar junction transistors, BJTs, consist of NPN or PNP “semiconductor sandwiches” as shown in Figure 1-10A. Note that leads are affixed to each of the three semiconductor regions. These regions are called the emitter, E, base, B, and collector, C, respectively. The boundary between P-type and N-type semiconductor materials is referred to as a junction. Consequently, BJTs are 2-junction, 3-terminal devices.

4. Figure 1-10A

5. Recall that a junction diode is formed by joining P-type and N-type semiconductor materials as illustrated previously in Figure 1-1 A. Consequently, BJTs can be modeled by two back-to-back diodes as shown in Figure 1-10B. Bipolar Transistors - Diode Model

6. Here, diode number 1 is called the emitter-base diode, while diode number 2 is referred to as the collector-base diode.

7. Bipolar Transistors - Schematic Symbol The schematic symbols for both NPN and PNP BJTs are shown in Figure 1-1OC. Notice that the only difference between the NPN and PNP symbols is the direction of the arrow, which indicates the emitter of the device. Specifically, the arrow points away from the base in the NPN symbol and toward the base in the PNP symbol. In Figure 1-10B and Figure 1-1OC, note that the current directions in the PNP transistor are just the opposite of those in the NPN transistor.

8. Figure 1-10C Bipolar Transistors - Schematic Symbol

9. The term biasing means establishing desired values of DC currents and voltages for an electronic device. In order to use a BJT for amplification, a biasing scheme must be employed so that: 1. The emitter-base diode is forward biased. 2. The collector-base diode is reverse biased. Bipolar Transistors - Biasing

10. Figure 1-11 illustrates one way to provide appropriate bias voltages for an NPN and PNP BJT. In both Figure 1-11A and Figure 1-11B, the polarity of V EE is such that the emitter-base diode is forward biased. Similarly, the polarity of V CC is such that the collector-base diode is reverse biased.

11. Figure 1-11A and B

12. Note in Fig. 1-11 that: The current directions and voltage polarities of the PNP circuit are just the opposite of those in the NPN circuit. Initially, we will concentrate on the analysis and design of NPN circuits. Once you have mastered NPN circuits, the transition to PNP circuits will be relatively easy. Bipolar Transistors - Biasing

13. BJT Circuit Configurations Many BJT circuits can be classified as common-base, common-emitter, or common-collector circuits as shown in Figure 1-12. Here, one lead is connected to the AC signal source, and a second lead provides the take-off point for the AC output voltage. The remaining, third, lead is called the “common” and identifies the particular configurations.

14. Figure 1-12A

15. Figure 1-12B

16. Figure 1-12C

17. Naturally, each configuration has its unique characteristics. These characteristics, and appropriate analysis and design methods will be discussed in detail in later units. BJT Circuit Configurations

18. Reverse Currents For each configuration in Figure 1-12, V CC reverse biases the collector-base diode of the BJT. Opening the emitter circuit of Figure 1-12A produces the circuit shown in Figure 1-13A. Here, the collector current is simply the reverse current of the collector-base diode. Manufacturers list this reverse current as I CBO on the transistor’s data sheet. Note that current flows between the BJT terminals identified by subscripts C and B. Similarly, the O subscript indicates that the emitter terminal is open.

19. Figure 1-13A

20. If you open the base circuit in Figure 1-12B you obtain the circuit shown in Figure 1-13B. Here, the collector current once again equals the reverse current of the collector-base diode. In this case, however, the reverse current is designated as I CEO since the base terminal is open. Reverse Currents

21. Figure 1-13B

22. N-type material is rich in negative charge carriers (electrons) and P-type material is rich in positive charge carriers (holes). Consequently, electrons and holes are referred to as majority carriers in N-type and P-type materials respectively. Since it is not possible to manufacture perfect semiconductor materials, N-type materials contain a small number of holes, and P-type materials contain a small number of free electrons. Basic Transistor Action

23. The electrons in the P-type material and the holes in the N-type materials are called minority carriers. Table 1-2 summarizes the concept of majority and minority carriers for N-type and P-type materials.

24. Basic Transistor Action When a PN junction is said to be forward or reverse biased, it is important to realize that the forward or reverse biased condition applies only to the majority carriers. From the point of view of the minority carriers, the situation is the opposite. Specifically: 1. A forward biased PN junction is reverse biased with respect to minority carriers. 2. A reverse biased PN junction is forward biased with respect to minority carriers.

25. Basic Transistor Action To illustrate statement 2 above, consider the circuit shown in Figure 1 -14. Here, the PN junction is reverse biased by the external voltage source, V CC. Consequently, majority carriers are prevented from crossing the junction. Note however, that the polarity of V CC is such that the minority carriers are forced across the junction. For this reason, a small reverse current, I R, flows in the circuit.

26. Figure 1-14

27. Basic Transistor Action Let’s examine the action that takes place in the properly biased NPN transistor shown in Figure 1-15.

28. Basic Transistor Action Here, the operation of the circuit can be summarized as follows: 1. The emitter base junction is forward biased by the external voltage source V EE. Consequently, a steady flow of electrons, supplied by V EE, is injected into the base region from the emitter region.

29. Basic Transistor Action 2. The collector base junction is reverse biased by the external voltage source, V CC. Since electrons are minority carriers in P-type materials, and since a reverse biased PN junction appears forward biased to minority carriers, most of the electrons injected into the base region are swept across the collector-base junction. Typically, 95 to 99 percent of the electrons supplied by the emitter flow through the collector region and into the external voltage source, V CC. The remaining 1 to 5 percent of the injected electrons combine with holes in the base region.

30. Basic Transistor Action This establishes a small base current which flows out of the base region and into the external circuit. In order for the action just described to occur, the various regions of the BJT must be specially constructed. Specifically: 1. The emitter region contains large numbers of majority carriers. This ensures that a large number of electrons will be supplied to the base region.

31. Basic Transistor Action 2. The base region is very thin and contains relatively few majority carriers. This is necessary to minimize the combining of holes and injected electrons in the base region. 3. The collector region contains moderate numbers of majority carriers, and is physically larger than either the base or emitter regions. This ensures adequate “collection" of the electrons swept across the collector-base junction.

32. Basic Transistor Action Although it is quite useful to model a BJT as two back-to-back diodes, the transistor action described previously does not occur if two discrete semiconductor diodes are placed back to back. This is because the P-type and N-type regions of semiconductor diodes do not satisfy the special requirements necessary for proper BJT operation.

33. BJT Formulas At this point, we will introduce a number of formulas that are useful for the analysis and design of BJT circuits. In addition, we will use approximations in order to simplify the formulas as much as possible. With reference to Figure 1-15 then: l E = l B + l C (Eq.1-9) Where: I E, I B, and I C are the DC currents in amperes, A, in the emitter, base, and collector leads respectively.

34. BJT Formulas In Figure 1-15, it is clear that internally, l E supplies current to both the base and collector regions. The ratio of I EC to I E is called alpha,, and indicates the portion of the emitter current, l E, that enters the collector region. Stated mathematically:

35. BJT Formulas Also, since I EC - I C - I CBO,, may be defined as: Where: I C and I E are as defined previously, I CBO = reverse current in amperes, A, of the collector-base diode. Since I C = I EC + l CBO, and I EC = I E l C = I E + I CBO (Eq.1-11)

36. BJT Formulas In Figure 1-15, note that I B = I E - I EC - I CBO. Substituting I E for I EC yields: I B = I E - I E - I CBO I B = I E (1 - ) - I CBO The ratio of I EC to I EB is defined as beta, B. Substituting I C - I CBO for I EC, and I B + l CBO for I EB yields:

37. BJT Formulas Solving Equation 1 -13 for the collector current, l C, yields: l C - I CBO = Bl B + BI CBO I C = Bl B + BI CBO + I CBO I C = BI B + I CBO (B + 1) (Eq.1-14)

38. BJT Formulas Occasionally the following identities will also prove useful:

39. BJT Formulas By using one or more of the previous identities, you can derive numerous equivalent relationships. For example, substituting Equation 1-17 into Equation 1-12 yields:

40. BJT Formulas If you examine typical BJT data sheets, you will find the following important facts: 1. Values of typically range from 0.95 to 0.99. 2. B rarely has a value less than 20. 3. I CBO is small compared to I C for silicon transistors. 4. (B + 1) l CBO is small compared to BI B in most circuits that use silicon transistors.

41. BJT Formulas Based upon the previous observations, the following approximations are frequently employed. ≂ 1 B+1 ≂ B l CBO ≂ 0

42. BJT Formulas By using one or more of these approximations, we can simplify most of the formulas given previously. Table 1-3 summarizes the original and resulting approximate formulas.

43. Table 1-3

44. In many cases, the approximate formulas in Table 1-3 are sufficiently accurate for the analysis and design of BJT circuits. Example 1 -5 A silicon BJT has a B of 100, and an I CBO of 0.01A. Calculate the value of, I C and I B assuming I E = 1mA. We will calculate the various quantities using both the original and approximate formulas. BJT Formulas

46. Now for the approximate formulas: Clearly, the values predicted by the approximate formulas are very close to the values predicted by the original formulas. As a guide, you can use the approximate formulas if:

47. BJT Formulas Example 1 -6 A germanium BJT has a B of 100 and an I CBO of IA. Should you use the original or approximate formulas, if I E is 1mA In this case: Since 10 is not > 20, you should use the original formulas. Generally speaking, germanium transistors have large values of ICBO compared to silicon transistors.

48. BJT DC Parameters The data sheet of a particular BJT provides minimum, typical and/or maximum values for a number of BJT parameters. In this course, appropriate parameters will be introduced as required. In this section, we will briefly discuss the most important DC parameters.

49. BJT DC Parameters MAXIMUM RATINGS Like any electrical device, BJTs will be damaged or destroyed if the current, voltage or power is excessive. Consequently, you should never exceed the maximum ratings, usually specified at 25°C. Above 25°C, you should use appropriate derating factors to determine the maximum rating at the elevated temperature as illustrated by the following example.

50. BJT DC Parameters Example 1-7 For a certain transistor, the following information is provided under the Maximum Ratings portion of the data sheet. What is the maximum power the transistor can dissipate at 50°C? Maximum RatingsSymbolValueUnit Total Power Dissipation @ 25°C PDPD 2Watt Derate Above 25°C D20 mW/°C

51. BJT DC Parameters The difference between 50°C and 25°C is obviously 25°C. Since the power must be derated by 20mW for each °C rise in temperature, we have: P = P 25°C - DΔT P = 2W - 20mW/°C(25°C) P = 2W - 0.5W = 1.5W

52. BJT DC Parameters DC CURRENT GAIN This is the parameter B introduced previously. Data sheets use the symbol h FE to indicate the value of B. Typically, h FE can vary by as much as 4:1 among transistors of the same type. In addition, for a given transistor, h FE varies with temperature, emitter current, and time.

53. BJT DC Parameters LEAKAGE CURRENTS I CBO, I CEO, and I EBO are the symbols used to represent the various leakage currents. Even though these currents are measured respectively with the emitter, base, and collector leads open, they can adversely affect the operation of a BJT circuit when the leads are not open. Data sheets almost always list the maximum value of I CBO.

54. BJT DC Parameters For transistors of the same type, I CBO can vary by as much as 100:1. In addition, for a given transistor, I CBO is sensitive to temperature changes. As a guide, I CBO doubles for, approximately, every 8°C rise in temperature.

55. BJT DC Parameters BASE-TO-EMITTER VOLTAGE For a given value of emitter current, the base-to-emitter voltage varies linearly by approximately -2mV to -2.5mV per C rise in temperature. Variations in V BE in turn produce changes in the emitter and collector current. Techniques for stabilizing BJT circuits against variations in h FE, I CBO, and V BE will be discussed in Unit 2.

56. BJT DC Parameters Fig. 1-16A BJT Input and Output Curves The circuit shown in Figure 1-16A can be used to obtain both input and output curves for an NPN transistor in the common-base configuration.

57. BJT DC Parameters To obtain input curves, you can use the following procedure: 1. Adjust R2 to obtain a particular value of V CB - for example 1V. 2. Adjust R1 to obtain various values of I E. For each value of I E, note the corresponding value of V BE. 3. Readjust R 2 to obtain a different value of V CB - for example 10V. Then repeat step 2 for the new value of V CB

58. BJT DC Parameters Fig. 1-16B In this manner, you can obtain data to plot the input curves shown in Figure 1-16B.

59. BJT DC Parameters Since the emitter-base diode of the BJT is forward biased, you should not be surprised that the input curves are virtually identical to the IV curve of a forward biased junction diode. In Figure 1-16B, note that the curve obtained when V CB = 10V differs only slightly from the curve obtained when V CB = 1V. Based on this observation, it should be clear that V CB has only a minor effect on the BJT’s input characteristics. Consequently, in most circuits, you can ignore the effects of V CB on the input characteristics of the BJT.

60. BJT DC Parameters Fig. 1-16C In order to obtain the output curves shown in Figure 1-16C, you can use the following procedure. 1. Adjust R1 to obtain a particular value of l E -for example 1 mA. 2. Adjust R2 to obtain various values of V CB. For each value of V CB, note the corresponding value of I C.

61. BJT DC Parameters Fig. 1-16C 3. Repeat steps 1 and 2 for different values of I E - for example 2mA, 3mA, 4mA, and 5mA.

62. BJT DC Parameters The output curves in Figure 1 -16C are in agreement with the approximate, formula I C = ⍺ I E. Since ⍺ is only slightly less than 1, it follows that I C should approximately equal I E. This suggests that the BJT's output characteristics are essentially those of a dependent current source. Specifically, once the emitter current is fixed, the collector current essentially remains constant for wide variations in the collector-to- base voltage, V CB.

63. BJT DC Parameters In Figure 1 -16C, note that V CB must be made slightly negative to completely reduce I C to zero. This is an unusual feature of a common-base circuit, and occurs because of an inherent collector-to-base junction potential within the BJT.

64. BJT DC Models Fig. 1-18A The input curves of a BJT indicate that the BJT acts like a forward biased junction diode, when viewed from the emitter-base terminals. Similarly, the BJT’s output curves indicate that the BJT acts like a current source, equal to l C, when viewed from the collector-base terminals.

65. BJT DC Models Fig. 1-18B For these reasons, a BJT can be represented by the DC equivalent circuit shown in Figure 1-18B.

66. BJT DC Models Here, note that: 1. The emitter-base portion of the BJT has been replaced by the large-signal model of a junction diode. 2. The collector-base portion of the BJT has been replaced by two shunt current sources. Recall that the collector current, I C equals ⍺ I E + l CBO Thus, each current source supplies one of the two components of the collector current.

67. BJT DC Models As discussed previously, l CBO is usually negligible compared to ⍺ I E. Also, in most circuits r F is negligible compared to the external circuit resistance.

68. BJT DC Models Figures 1-18B and 1-18C Consequently, the equivalent circuit in Figure 1-18B can often be simplified as shown in Figure 1-18C. Here, it is assumed that ⍺ = 1, and that the transistor is a silicon transistor since V T = 0.7V.

69. BJT DC Models In Figure 1-16A, the polarity of V CB is such that the collector-base diode is reverse biased. Consequently, as V CB is increased, eventually a point is reached where the collector base diode breaks down. Once this point is reached, the collector current increases sharply, and can destroy the transistor. The value of this collector break down voltage is designated as BV CBO or V CBO.

70. BJT DC Models Obviously, in a given circuit, V CB should never be permitted to equal or exceed the value of BV CBO. When I E = 0, you have a situation that is essentially equivalent to an open emitter lead. For this reason, when I E = 0, I C = l CBO as shown by the bottom curve in Figure 1-16C. For clarity, the value of I CBO has been exaggerated on the bottom curve in Figure 1 -16C.

71. BJT DC Models Fig. 1-17A A circuit that can be used to obtain both input and output curves for an NPN transistor in the common-emitter configuration is shown in Figure 1-17A.

72. BJT DC Models Fig. 1-17B and 1-17C By following a procedure similar to the one used for the common-base circuit, you can obtain the input and output curves shown respectively in Figure 1-17B, and Figure 1-17C.

73. BJT DC Models Once again, the input curves are essentially those of a forward biased junction diode. The output curves in Figure 1-17C are in good agreement with the approximate formula I C = BI B. For example, since B is 100, you would expect l C to be 10mA when I E is 100A. Similarly, when I B = 80A, I C should be about 8mA. These calculations agree with the curves in Figure 1 -17C.

74. BJT DC Models Fig. 1-19A The current sources in Figure 1-18B generate currents equal to ⍺ I E and l CBO so that I C = ⍺ I E + l CBO. Recall that an equally valid expression for the collector current, l C, is l C = BI B + I CEO

75. BJT DC Models Fig. 1-19B Consequently, a BJT can also be modeled as shown in Figure 1-19B.

76. BJT DC Models Fig. 1-19C Furthermore, if the effects of r F and l CEO are negligible, the simplified model of Figure 1-19C can be used to represent the DC equivalent circuit of the BJT.

77. BJT DC Models Fig. 1-20A Example 1-8 For the circuit shown in Figure 1-20A, estimate the values of I E, I C and V CB. Assume the effects of r F and I CBO are negligible.

78. BJT DC Models Fig. 1-20B Using the simplified BJT model of Figure 1-18C, the DC equivalent circuit is sketched as shown in Figure 1-20B.

79. BJT DC Models Here: The voltage between the collector and base, V CB, equals the collector supply voltage, 15V, minus the drop across the 5kΩ resistor. Thus: V CB = 15V -1mA(5kΩ)= 10V

80. BJT DC Models Fig. 1-21A Example 1 -9 Estimate the values I B, I C and V CE for the circuit shown in Figure 1-21A.

81. BJT DC Models Fig. 1-21B The circuit in Figure 1-21A is drawn with “ground referenced notation”, in other words, the way it is conventionally drawn on a schematic diagram. For clarity, we have redrawn the circuit as shown in Figure 1-21 B.

82. BJT DC Models Since the value of B is given, it is convenient to use the simplified BJT DC model of Figure 1- 19C. Naturally we are assuming the effects of r F and l CBO are negligible. If this were not the case, you could use the BJT DC model of Figure 1-19B to analyze the circuit.

83. BJT DC Models Fig. 1-21C The DC equivalent circuit is provided in Figure 1 -21C.

84. BJT DC Models Here: The voltage between the collector and emitter, V CE, equals the collector supply voltage, 20V, minus the drop across the 10kΩ resistor. Thus: V CE = 20V - 1.5mA(10kΩ) = 5v.

85. Self Test Review Question 1 The emitter base junction of a BJT is normally ___________ biased, and the collector-base junction is ________ biased.

86. Answer to Question 1 forward, reverse

87. Self Test Review Question 2 I CEO is (larger/smaller) then I CBO

88. Answer to Question 2 larger

89. Self Test Review Question 3 Electrons are __________ carriers in P-type materials.

90. Answer to Question 3 minority

91. Self Test Review Question 4 On a data sheet the symbol ___________ represents B.

92. Answer to Question 4 h FE

93. Self Test Review Question 5 ⍺ is approximately equal to ______________.

94. Answer to Question 5 1

95. Self Test Review Question 6 When viewed from the emitter-base terminals, a BJT acts like a ____________.

96. Answer to Question 6 forward biased diode

97. Self Test Review Question 7 When viewed from the collector-base terminals, a BJT acts like a _____________ source.

98. Answer to Question 7 current

99. Self Test Review Question 8 If the ⍺ of a BJT changes from 0.98 to 0.99, B will change from ________ to __________

100. Answer to Question 8 49 to 99

101. Self Test Review Question 9 For a given type transistor B typically varies over a ___________ range.

102. Answer to Question 9 4:1

103. Self Test Review Question 10 If the base lead of a BJT is open I C = ______________.

104. Answer to Question 10 l CEO

105. Summary Junction diodes are formed by joining P-type and N-type semiconductor materials. The DC, or large-signal model of a diode is represented by the series combination of an ideal diode, the diode’s forward resistance, r F, and the diode's turn-on voltage V T. When a diode is simultaneously driven from a large DC and a small AC source, the diode acts like a resistance to the AC source. This AC, dynamic, resistance consists of two components - bulk resistance, r B, and junction resistance, r j.

106. Summary You can estimate a diode’s AC resistance by using the following formula: When you analyze diode and transistor circuits, it is often necessary to obtain DC and AC equivalent circuits. In the DC equivalent circuit, capacitors are replaced by open circuits, and. the device is replaced by its large-signal model.

107. Summary Similarly, in the AC equivalent circuit, capacitors are replaced by short circuits, and the device is replaced by its AC, or small-signal, model. You can use graphical methods, using Ioad lines, to analyze diode, and transistor circuits. The intersection between the DC load line, and the devices characteristic curve establishes the DC or quiescent operating point. The AC load line is useful for determining how a device responds to AC signals. You will encounter applications for load lines in later units.

108. Summary BJTs consist of NPN or PNP “semiconductor sandwiches”. For normal operation of a BJT, the emitter-base junction is forward biased, and the collector- base junction is reverse biased. Consequently, the current directions and voltage polarities in PNP circuits are just the opposite of those encountered in NPN circuits. Frequently, BJT circuits are classified as common-base, common-emitter, or common- collector circuits.

109. Summary In each case, one lead is connected to the AC signal source, and one lead is used as the take- off point for the AC output voltage. The remaining lead is termed the “common”, and identifies the particular configuration. BJTs operate by the processes of injection, diffusion, and collection. The forward biased emitter-base junction majority carriers are injected from the emitter region into the base region.

110. Summary Since the base region is thin and has few of its own majority carriers, the injected carriers rapidly diffuse, or spread out, in the base region. Finally, most of the injected carriers are swept across the reverse biased collector-base junction where they are effectively collected by the relatively large collector region. Various parameters and formulas used to describe the operation of BJTs were introduced in the unit. Important parameters include ⍺, B or h FE, V BE, I CBO, l CEO, BV CBO, and BV CEO.

111. Summary Definitions for these parameters are provided in appropriate sections in the unit. Many of these parameters vary widely from one BJT to another. In addition, V BE, l CBO, l CEO, and h FE are very temperature sensitive. Table 1-3 summarizes the most frequently used BJT DC formulas. By using the formulas for I C given in Table 1-3, two equivalent DC models for the BJT were developed. Examples illustrating the use of each model are provided in the unit.