1. SJTU Zhou Lingling1 Chapter3 Bipolar Junction Transistor (BJT)

2. SJTU Zhou Lingling2 Outline Introduction Operation in the Active Mode Analysis of Transistor Circuits at DC The transistor as an Amplifier Graphical Analysis Biasing the BJT for Discrete-Circuit Design Configuration for Basic Single Stage BJT Amplifier High frequency Model

3. SJTU Zhou Lingling3 Physical Structure Circuit Symbols for BJTs Modes of Operation Basic Characteristic Introduction

4. SJTU Zhou Lingling4 Physical Structure A simplified structure of the npn transistor.

5. SJTU Zhou Lingling5 Physical Structure A dual of the npn is called pnp type. This is the simplified structure of the pnp transistor.

6. SJTU Zhou Lingling6 Circuit Symbols for BJTs The emitter is distinguished by the arrowhead.

7. SJTU Zhou Lingling7 Modes of Operation ModesEBJCBJApplication CutoffReverse Switching application in digital circuits SaturationForward ActiveForwardReverseAmplifier Reverse active ReverseForward Performance degradation

8. SJTU Zhou Lingling8 Basic Characteristics Far more useful than two terminal devices (such as diodes) The voltage between two terminals can control the current flowing in the third terminal. We can say that the collector current can be controlled by the voltage across EB junction. Much popular application is to be an amplifier

9. SJTU Zhou Lingling9 Operation in the Active Mode Current flow Current equation Graphical representation of transistor’s characteristics

10. SJTU Zhou Lingling10 Current Flow Current flow in an npn transistor biased to operate in the active mode.

11. SJTU Zhou Lingling11 Collector Current Collector current is the drift current. Carriers are successful excess minority carriers. The magnitude of collector current is almost independent of voltage across CB junction. This current can be calculated by the gradient of the profile of electron concentration in base region.

12. SJTU Zhou Lingling12 Base Current Base current consists of two components.  Diffusion current  Recombination current Recombination current is dominant. The value of base current is very small.

13. SJTU Zhou Lingling13 Emitter Current Emitter current consists of two components. Both of them are diffusion currents. Heavily doped in emitter region. Diffusion current produced by the majority in emitter region is dominant.

14. SJTU Zhou Lingling14 Profiles of Minority-Carrier Concentrations

15. SJTU Zhou Lingling15 Current Equation Collector current Base current Emitter current

16. SJTU Zhou Lingling16 Explanation for Saturation Current Saturation current is also called current scale. Expression for saturation current: Has strong function with temperature due to intrinsic carrier concentration. Its value is usually in the range of 10 -12 A to 10 -18 A.

17. SJTU Zhou Lingling17 Explanation for Common-Emitter Current Gain Expression for common –emitter current gain: Its value is highly influenced by two factors. Its value is in the range 50 to 200 for general transistor.

18. SJTU Zhou Lingling18 Explanation for Common-Base Current Gain Expression for common –base current gain: Its value is less than but very close to unity. Small changes in α correspond to very large changes in β.

19. SJTU Zhou Lingling19 Recapitulation Collector current has the exponential relationship with forward-biased voltage as long as the CB junction remains reverse- biased. To behave as an ideal constant current source. Emitter current is approximately equal to collector current.

20. SJTU Zhou Lingling20 Graphical Representation of Transistor’s Characteristics Characteristic curve relates to a certain configuration. Input curve is much similar to that of the diode, only output curves are shown here. Three regions are shown in output curves. Early Effect is shown in output curve of CE configuration.

21. SJTU Zhou Lingling21 Output Curves for CB Configuration

22. SJTU Zhou Lingling22 Output Curves for CB Configuration Active region  EBJ is forward-biased, CBJ is reverse-biased;  Equal distance between neighbouring output curves;  Almost horizontal, but slightly positive slope. Saturation region  EBJ and CBJ are not only forward-biased but also turned on;  Collector current is diffusion current not drift current.  Turn on voltage for CBJ is smaller than that of EBJ. Breakdown region  EBJ forward-biased, CBJ reverse-biased;  Great voltage value give rise to CBJ breakdown;  Collector current increases dramatically.

23. SJTU Zhou Lingling23 Output Curves for CE Configuration (a) Conceptual circuit for measuring the i C – v CE characteristics of the BJT. (b) The i C – v CE characteristics of a practical BJT.

24. SJTU Zhou Lingling24 The Early Effect Curves in active region are more sloped than those in CB configuration. Early voltage. Effective base width and base width modulation.

25. SJTU Zhou Lingling25 The Early Effect(cont’d) Assuming current scale remains constant, collector current is modified by this term: Narrow base width, small value of Early voltage, strong effect of base width modulation, strong linear dependence of on.

26. SJTU Zhou Lingling26 Analysis of Transistor Circuit at DC Equivalent Circuit Models Analysis Steps Examples

27. SJTU Zhou Lingling27 Equivalent Circuit Models Large-signal equivalent-circuit models of the npn BJT operating in the forward active mode. In practical DC analysis, constant voltage drop model is popular used.

28. SJTU Zhou Lingling28 DC Analysis Steps a.Using simple constant-voltage drop model, assuming, irrespective of the exact value of currents. b.Assuming the device operates at the active region, we can apply the relationship between I B, I C, and I E, to determine the voltage V CE or V CB. c.Check the value of V CE or V CB, if i.V C >V B (or V CE >0.2V), the assumption is correct. ii.V C <V B (or V CE <0.2V), the assumption is incorrect. It means the BJT is operating in saturation region. Thus we shall assume V CE =V CE(sat) to obtain I C. Here the common emitter current gain is defined as  forced =I C /I B, we will find  forced < .

29. SJTU Zhou Lingling29 Example 5.4 (a)circuit; (b)circuit redrawn to remind the reader of the convention used in this book to show connections to the power supply

30. SJTU Zhou Lingling30 Example 5.4 analysis with the steps numbered.

31. SJTU Zhou Lingling31 Example 5.5 This circuit is identical to that considered in Example 5.4 except that now the base voltage is 6V.

32. SJTU Zhou Lingling32 Example 5.5

33. SJTU Zhou Lingling33 Example 5.10 Assume, to determine the voltages at all nodes and the currents through all branches.

34. SJTU Zhou Lingling34 Example 5.10

35. SJTU Zhou Lingling35 Example 5.10

36. SJTU Zhou Lingling36 Example 5.11

37. SJTU Zhou Lingling37 Example 5.11

38. SJTU Zhou Lingling38 Example 5.12

39. SJTU Zhou Lingling39 Example 5.12

40. SJTU Zhou Lingling40 Examples Example 5.4 shows the order of the analysis steps indicated by the circled numbers. Example 5.5 shows the analysis of BJT operating saturation mode. Example 5.6 shows the transistor operating in cutoff mode.

41. SJTU Zhou Lingling41 Examples(cont’d) Example 5.7 shows the analysis for pnp type circuit. It indicates the the current is affected by ill- specified parameter β. As a rule, one should strive to design the circuit such that its performance is as insensitive to the value of β as possible. Example 5.8 is the bad design due to the currents critically depending on the value of β. Example 5.9 is similar to the example 5.5 except the transistor is pnp type.

42. SJTU Zhou Lingling42 Examples(cont’d) Example 5.10 shows the application of Thévenin’s theorem in calculating emitter current and so on. This circuit is the good design for the emitter is almost independent of β and temperature. Example 5.11 shows the DC analysis for two stage amplifier. Example 5.12 shows the analysis of the power amplifier composed of the complimentary transistors.

43. SJTU Zhou Lingling43 The Transistor as an Amplifier Conceptual Circuits Small-signal equivalent circuit models Application of the small-signal equivalent circuit models Augmenting the hybrid π model.

44. SJTU Zhou Lingling44 Conceptual Circuit (a) Conceptual circuit to illustrate the operation of the transistor as an amplifier. (b) The circuit of (a) with the signal source v be eliminated for dc (bias) analysis.

45. SJTU Zhou Lingling45 Conceptual Circuit(cont’d) With the dc sources (V BE and V CC ) eliminated (short circuited), thus only the signal components are present. Note that this is a representation of the signal operation of the BJT and not an actual amplifier circuit.

46. SJTU Zhou Lingling46 Small-Signal Circuit Models Transconductance Input resistance at base Input resistance at emitter Hybrid π and T model

47. SJTU Zhou Lingling47 Transconductance Expression Physical meaning g m is the slope of the i C – v BE curve at the bias point Q. At room temperature,

48. SJTU Zhou Lingling48 Input Resistance at Base and Emitter Input resistance at base Input resistance at emitter Relationship between these two resistances

49. SJTU Zhou Lingling49 The Hybrid-  Model The equivalent circuit in (a) represents the BJT as a voltage-controlled current source (a transconductance amplifier), The equivalent circuit in (b) represents the BJT as a current-controlled current source (a current amplifier).

50. SJTU Zhou Lingling50 The T Model These models explicitly show the emitter resistance r e rather than the base resistance r  featured in the hybrid-  model.

51. SJTU Zhou Lingling51 Augmenting the Hybrid-  Model  Expression for the output resistance.  Output resistance represents the Early Effect(or base width modulation)

52. SJTU Zhou Lingling52 Models for pnp Type Models derived from npn type transistor apply equally well to pnp transistor with no changes of polarities. Because the small signal can not change the bias conditions, small signal models are independent of polarities. No matter what the configuration is, model is unique. Which one to be selected is only determined by the simplest analysis.

53. SJTU Zhou Lingling53 Graphical Analysis a.Graphical construction for the determination of the dc base current in the circuit. b.Load line intersects with the input characteristic curve.

54. SJTU Zhou Lingling54 Graphical Analysis(cont’d) Graphical construction for determining the dc collector current I C and the collector-to-emitter voltage V CE in the circuit.

55. SJTU Zhou Lingling55 Small Signal Analysis Graphical determination of the signal components v be, i b, i c, and v ce when a signal component v i is superimposed on the dc voltage V BB

56. SJTU Zhou Lingling56 Effect of Bias-Point Location on Allowable Signal Swing a.Load-line A results in bias point Q A with a corresponding V CE which is too close to V CC and thus limits the positive swing of v CE. b.At the other extreme, load-line B results in an operating point too close to the saturation region, thus limiting the negative swing of v CE.

57. SJTU Zhou Lingling57 Biasing in BJT Amplifier Circuit Biasing with voltage  Classical discrete circuit bias arrangement  Single power supply  Two-power-supply  With feedback resistor Biasing with current source

58. SJTU Zhou Lingling58 Classical Discrete Circuit Bias Arrangement by fixing V BE by fixing I B.

59. SJTU Zhou Lingling59 Classical Discrete Circuit Bias Arrangement Both result in wide variations in I C and hence in V CE and therefore are considered to be “bad.” Neither scheme is recommended.

60. SJTU Zhou Lingling60 Classical Biasing for BJTs Using a Single Power Supply  Circuit with the voltage divider supplying the base replaced with its Thévenin equivalent.  Stabilizing the DC emitter current is obtained by considering the negative feedback action provided by R E

61. SJTU Zhou Lingling61 Classical Biasing for BJTs Using a Single Power Supply Two constraints Rules of thumb

62. SJTU Zhou Lingling62 Two-Power-Supply Version Resistor R B can be eliminated in common base configuration. Resistor R B is needed only if the signal is to be capacitively coupled to the base. Two constraints should apply.

63. SJTU Zhou Lingling63 Biasing with Feedback Resistor  Resistor R B provides negative feedback.  I E is insensitive to β provided that  The value of R B determines the allowable signal swing at the collector.

64. SJTU Zhou Lingling64 Biasing Using Current Source (a)Q 1 and Q 2 are required to be identical and have high β. (b)Short circuit between Q 1 ’s base and collector terminals. (c)Current source isn’t ideal due to finite output resistor of Q 2

65. SJTU Zhou Lingling65 Application of the Small-Signal Models a.Determine the DC operating point of BJT and in particular the DC collector current I C (I CQ ). b.Calculate the values of the small-signal model parameters, such as g m =I C /V T, r  =  /g m =V T /I B, r e =  /g m =V T /I E. c.Draw ac circuit path. d.Replace the BJT with one of its small-signal models. The model selected may be more convenient than the others in circuits analysis. e.Determine the required quantities.

66. SJTU Zhou Lingling66 Basic Single-Stage BJT Amplifier Characteristic parameters Basic structure Configuration  Common-Emitter amplifier  Emitter directly connects to ground  Emitter connects to ground by resistor R E  Common-base amplifier  Common-collector amplifier(emitter follower)

67. SJTU Zhou Lingling67 Characteristic Parameters of Amplifier  This is the two-port network of amplifier.  Voltage signal source.  Output signal is obtained from the load resistor.

68. SJTU Zhou Lingling68 Definitions Input resistance with no load Input resistance Open-circuit voltage gain Voltage gain

69. SJTU Zhou Lingling69 Definitions(cont’d) Short-circuit current gain Current gain Short-circuit transconductance

70. SJTU Zhou Lingling70 Definitions(cont’d) Open-circuit overall voltage gain Overall voltage gain

71. SJTU Zhou Lingling71 Definitions(cont’d) Output resistance of amplifier properOutput resistance

72. SJTU Zhou Lingling72 Definitions(cont’d) Voltage amplifier Transconductance amplifier Voltage amplifier

73. SJTU Zhou Lingling73 Relationships Voltage divided coefficient

74. SJTU Zhou Lingling74 Basic Structure Basic structure of the circuit used to realize single-stage, discrete-circuit BJT amplifier configurations.

75. SJTU Zhou Lingling75 Common-Emitter Amplifier

76. SJTU Zhou Lingling76 Common-Emitter Amplifier Equivalent circuit obtained by replacing the transistor with its hybrid-  model.

77. SJTU Zhou Lingling77 Characteristics of CE Amplifier Input resistance Voltage gain Overall voltage gain Output resistance Short-circuit current gain

78. SJTU Zhou Lingling78 Summary of CE amplifier Large voltage gain Inverting amplifier Large current gain Input resistance is relatively low. Output resistance is relatively high. Frequency response is rather poor.

79. SJTU Zhou Lingling79 The Common-Emitter Amplifier with a Resistance in the Emitter

80. SJTU Zhou Lingling80 The Common-Emitter Amplifier with a Resistance in the Emitter

81. SJTU Zhou Lingling81 Characteristics of the CE Amplifier with a Resistance in the Emitter Input resistance Voltage gain Overall voltage gain Output resistance Short-circuit current gain

82. SJTU Zhou Lingling82 Summary of CE Amplifier with R E The input resistance R in is increased by the factor (1+g m R e ) The voltage gain from base to collector is reduced by the factor (1+g m R e ). For the same nonlinear distortion, the input signal v i can be increased by the factor (1+g m R e ). The overall voltage gain is less dependent on the value of β.

83. SJTU Zhou Lingling83 Summary of CE Amplifier with R E The reduction in gain is the price for obtaining the other performance improvements. Resistor R E introduces the negative feedback into the amplifier. The high frequency response is significant improved.

84. SJTU Zhou Lingling84 Common-Base Amplifier

85. SJTU Zhou Lingling85 Common-Base Amplifier

86. SJTU Zhou Lingling86 Characteristics of CB Amplifier Input resistance Voltage gain Overall voltage gain Output resistance Short-circuit current gain

87. SJTU Zhou Lingling87 Summary of the CB Amplifier Very low input resistance High output resistance Short-circuit current gain is nearly unity High voltage gain Noninverting amplifier Current buffer Excellent high-frequency performance

88. SJTU Zhou Lingling88 The Common-Collector Amplifier or Emitter-Follower

89. SJTU Zhou Lingling89 The Common-Collector Amplifier or Emitter-Follower

90. SJTU Zhou Lingling90 The Common-Collector Amplifier or Emitter-Follower

91. SJTU Zhou Lingling91 Characteristics of CC Amplifier Input resistance Voltage gain Overall voltage gain Output resistance Short-circuit current gain

92. SJTU Zhou Lingling92 Summary for CC Amplifier or Emitter-Follower High input resistance Low output resistance Voltage gain is smaller than but very close to unity Large current gain The last or output stage of cascade amplifier Frequency response is excellent well

93. SJTU Zhou Lingling93 Summary and Comparisons The CE configuration is the best suited for realizing the amplifier gain. Including R E provides performance improvements at the expense of gain reduction. The CB configuration only has the typical application in amplifier. Much superior high-frequency response. The emitter follower can be used as a voltage buffer and exists in output stage of a multistage amplifier.

94. SJTU Zhou Lingling94 Internal Capacitances of the BJT and High Frequency Model Internal capacitance  The base-charging or diffusion capacitance  Junction capacitances  The base-emitter junction capacitance  The collector-base junction capacitance High frequency small signal model Cutoff frequency and unity-gain frequency

95. SJTU Zhou Lingling95 The Base-Charging or Diffusion Capacitance Diffusion capacitance almost entirely exists in forward-biased pn junction Expression of the small-signal diffusion capacitance Proportional to the biased current

96. SJTU Zhou Lingling96 Junction Capacitances The Base-Emitter Junction Capacitance The collector-base junction capacitance

97. SJTU Zhou Lingling97 The High-Frequency Hybrid-  Model Two capacitances C π and C μ, where One resistance r x. Accurate value is obtained form high frequency measurement.

98. SJTU Zhou Lingling98 The Cutoff and Unity-Gain Frequency Circuit for deriving an expression for According to the definition, output port is short circuit

99. SJTU Zhou Lingling99 The Cutoff and Unity-Gain Frequency(cont’d) Expression of the short-circuit current transfer function Characteristic is similar to the one of first- order low-pass filter

100. SJTU Zhou Lingling100 The Cutoff and Unity-Gain Frequency (cont’d)

101. SJTU Zhou Lingling101 The High-Frequency Response equivalent circuit

102. SJTU Zhou Lingling102 The High-Frequency Response The circuit of (a) simplified at both the input side and the output side

103. SJTU Zhou Lingling103 The High-Frequency Response Equivalent circuit with C  replaced at the input side with the equivalent capacitance C eq

104. SJTU Zhou Lingling104 The High-Frequency Response Sketch of the frequency-response plot, which is that of a low-pass STC circuit.