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Chapter 4 Bipolar Junction Transistor BJT

2012/1/28. 2. J.P. Chen. Introduction. Three-terminal devices are far more useful than two-terminal ones, because they can be used in a multitude of applications, signal amplificationdigital logic memory circuits.

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Chapter 4 Bipolar Junction Transistor BJT

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    1. SJTU J. Chen 1 2012/1/28 Chapter 4 Bipolar Junction Transistor (BJT)

    2. 2012/1/28 2 J.P. Chen Introduction Three-terminal devices are far more useful than two-terminal ones, because they can be used in a multitude of applications, signal amplification digital logic memory circuits. …… The basic principle ? the use of the voltage between two terminals to control the current flowing in the third terminal. Transistor = transfer resistor

    3. 2012/1/28 3 J.P. Chen Part 1. Physical Operation and Current-Voltage Characteristics. Part 2. DC analysis of transistor circuits;Small Signal Operation and Models Part 3. The basic single-stage BJT amplifier and configurations Content We divide this chapter into three parts. In this chapter, we shall start with a simple description of the physical operation of the BJT. The main objective of this chapter is to develop the reader a high degree of familiarity with the BIT. Thus, by the end of the chapter, the reader should be able to perform rapid first-order analysis of transistor circuits and to design single-stage transistor amplifiers We divide this chapter into three parts. In this chapter, we shall start with a simple description of the physical operation of the BJT. The main objective of this chapter is to develop the reader a high degree of familiarity with the BIT. Thus, by the end of the chapter, the reader should be able to perform rapid first-order analysis of transistor circuits and to design single-stage transistor amplifiers

    4. SJTU J. Chen 4 2012/1/28 Part 1. Physical Operation and Current-Voltage Characteristics.

    5. 2012/1/28 5 J.P. Chen Physical Structure Circuit Symbols for BJTs Modes of Operation Basic Characteristic Introduction Though simple, this physical description provides considerable insight regarding the perfor- mance of the transistor as a circuit element. We then quickly move from describing current flow in terms of electrons and holes to a study of the transistor terminal characteristics. Though simple, this physical description provides considerable insight regarding the perfor- mance of the transistor as a circuit element. We then quickly move from describing current flow in terms of electrons and holes to a study of the transistor terminal characteristics.

    6. 2012/1/28 6 J.P. Chen Transistor Transistor is a three-terminal device, made of silicon (Germanium was early used) There are two types: NPN and PNP

    7. 2012/1/28 7 J.P. Chen Circuit symbol of the transistor A very descriptive and convenient circuit symbol exists for the BJT, The polarity of the device –npn and pnp is indicated by the direction of the arrowhead on the emitter.A very descriptive and convenient circuit symbol exists for the BJT, The polarity of the device –npn and pnp is indicated by the direction of the arrowhead on the emitter.

    8. 2012/1/28 8 J.P. Chen Physical Structure emitter region (n type) base region (p type) collector region (n type) About the physical structure, we should remember 3,3,2. A terminal is connected to each of the three semiconductor regions of the transistor, with the terminals labeled emitter (E), base (B), and collector (C). The transistor consists of two pn junctions, the emitter-base junction (EEJ) and the collector-base junction (CBJ). About the physical structure, we should remember 3,3,2. A terminal is connected to each of the three semiconductor regions of the transistor, with the terminals labeled emitter (E), base (B), and collector (C). The transistor consists of two pn junctions, the emitter-base junction (EEJ) and the collector-base junction (CBJ).

    9. 2012/1/28 9 J.P. Chen Structure of Actual Transistors the emitter region: highly doped the base region: very thin lightly doped the collector region: large area

    10. 2012/1/28 10 J.P. Chen Operation Modes Depending on the bias condition (forward or reverse) of each of the two junctions, there are different modes of operation of the BJT. In this course, we are mainly interested in the amplification mode.

    11. 2012/1/28 11 J.P. Chen About the Transistor A transistor can be used to realize a controlled source. In the extreme, the current in the third terminal can be changed from zero to a large value ? switch application. How is a BJT used to realize a controlled source or for a switch application? we should know that the operation process of BJT, especially in different operation modes. Let’s start by considering the physical operation of the transistor in the active mode.How is a BJT used to realize a controlled source or for a switch application? we should know that the operation process of BJT, especially in different operation modes. Let’s start by considering the physical operation of the transistor in the active mode.

    12. 2012/1/28 12 J.P. Chen Bias in Amplification Mode EBJ is forwardly biased CBJ is reversely biased Let’s start by considering the physical operation of the transistor in the active mode or amplification mode. Let’s start by considering the physical operation of the transistor in the active mode or amplification mode.

    13. 2012/1/28 13 J.P. Chen Current Flow in a NPN Transistor Two external voltage sources (shown as batteries) are used to establish the required bias conditions for active-mode operation. VBE make EBJ forward-biasing. VCE make CBJ reverse-baising.Two external voltage sources (shown as batteries) are used to establish the required bias conditions for active-mode operation. VBE make EBJ forward-biasing. VCE make CBJ reverse-baising.

    14. 2012/1/28 14 J.P. Chen Practical Source Connection

    15. 2012/1/28 15 J.P. Chen Current Flow in the Transistor

    16. 2012/1/28 16 J.P. Chen Profiles of Minority-Carrier Concentrations Let us now consider the electrons injected from the emitter into the base. These electrons will be minority carriers in the p-type base region. Because the base is usually very thin, in the steady state the excess minority-carrier (electron) concentration in the base will have an almost-straight-line profile, as indicated by the solid straight line in Fig. Let us now consider the electrons injected from the emitter into the base. These electrons will be minority carriers in the p-type base region. Because the base is usually very thin, in the steady state the excess minority-carrier (electron) concentration in the base will have an almost-straight-line profile, as indicated by the solid straight line in Fig.

    17. 2012/1/28 17 J.P. Chen Current Equation Terminal current relationship From above analysis, we can compute the three terminals’ currentFrom above analysis, we can compute the three terminals’ current

    18. 2012/1/28 18 J.P. Chen Collector current Collector current is the drift current. 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 where Is is the saturation current, also called current scale factor.

    19. 2012/1/28 19 J.P. Chen Expression for saturation current: Saturation Current The saturation current Is is inversely proportional to the base width W and is directly proportional to the area of the EBJ. The saturation current Is is inversely proportional to the base width W and is directly proportional to the area of the EBJ.

    20. 2012/1/28 20 J.P. Chen The saturation current is a strong function with temperature due to intrinsic carrier concentration. Its value is usually in the range of 10-12A to 10-18A. Saturation Current

    21. 2012/1/28 21 J.P. Chen Base Current Base current consists of two components. Diffusion current Recombination current Recombination current is dominant. The value of base current is very small.

    22. 2012/1/28 22 J.P. Chen Expression for common –emitter current gain: Common-Emitter Current Gain Finally, we note that the discussion thus far assumes an idealized situation, where ß is a constant for a given transistor. Finally, we note that the discussion thus far assumes an idealized situation, where ß is a constant for a given transistor.

    23. 2012/1/28 23 J.P. Chen A high ? is highly desirable since it represents a gain parameter In order to obtain a high ?, the base region should be thin and lightly doped and the emitter region heavily doped. ? is in the range 50 to 200 for general transistors Common-Emitter Current Gain

    24. 2012/1/28 24 J.P. Chen Emitter current consists of two components. Both of them are diffusion currents. Diffusion current produced by the majority in emitter region is dominant due to heavy doping in emitter region Emitter Current

    25. 2012/1/28 25 J.P. Chen Explanation of the Physical Structure highly doping in the emitter region and forward biased EBJ:

    26. 2012/1/28 26 J.P. Chen Explanation of the Physical Structure very thin base region: Much sharper gradient Much less recombination large area the collector region and reverse biased CBJ: Much more electrons emitted from the emitter region can be “captured” by the strong electric field in the reverse CBJ into the collector region

    27. 2012/1/28 27 J.P. Chen CBJ and reversely biased diode The reversely biased CBJ behaves totally different from a reversely biased single diode, where almost no current flows. The huge difference arises from the minority concentration! In a BJT, the minority concentration in the Base region is extraordinarily high due to electrons injection from the Emitter region

    28. 2012/1/28 28 J.P. Chen Terminal current relationship

    29. 2012/1/28 29 J.P. Chen Expression for common-base current gain: Its value is less than but very close to unity. Small changes in a correspond to very large changes in ß. Common-Base Current Gain

    30. 2012/1/28 30 J.P. Chen Collector current has the exponential relationship with forward-biased voltage VBE as long as the CB junction remains reverse-biased. It behaves as an ideal constant current source (independent of VCB or VCE). Emitter current is approximately equal to collector current. Recapitulation

    31. 2012/1/28 31 J.P. Chen Graphical representation The ic-vBE characteristic for an npn transistor It is sometimes useful to describe the transistor i-v characteristics graphically, especially for qualitative analysis. Figure shows the ic-v BE characteristic, which is the exponential relationship. When vBE>0.7v, the curve rises very sharply. When vBE<0.5v, the current is negligibly small. In performing rapid first-order dc calculations we normally will assume that V BE = 0.7 V, As in silicon diodes, the voltage across the emitter-base junction decreases by about 2 m V for each rise of 1°C in temperature, Figure illustrates this temperature dependence by depicting ic-v BE curves at three different temperatures for an npn transistor. We can find a rule: when the temperature increases, the curve will drift to left.It is sometimes useful to describe the transistor i-v characteristics graphically, especially for qualitative analysis. Figure shows the ic-v BE characteristic, which is the exponential relationship. When vBE>0.7v, the curve rises very sharply. When vBE<0.5v, the current is negligibly small. In performing rapid first-order dc calculations we normally will assume that V BE = 0.7 V, As in silicon diodes, the voltage across the emitter-base junction decreases by about 2 m V for each rise of 1°C in temperature, Figure illustrates this temperature dependence by depicting ic-v BE curves at three different temperatures for an npn transistor. We can find a rule: when the temperature increases, the curve will drift to left.

    32. 2012/1/28 32 J.P. Chen Dependence of Ic on VCB Figure shows the iC versus vCB characteristics of an npn transistor for various values of the emitter current iE. As can be seen, the curves are horizontal straight lines. which show the collector behaves as a constant-current Source. Ic=aiE, in this case, the transistor may be thought of a current-controlled current source.Figure shows the iC versus vCB characteristics of an npn transistor for various values of the emitter current iE. As can be seen, the curves are horizontal straight lines. which show the collector behaves as a constant-current Source. Ic=aiE, in this case, the transistor may be thought of a current-controlled current source.

    33. 2012/1/28 33 J.P. Chen The Early Effect When operated in the active region, dependence of the collector current on the voltage exists, ic versus vCE characteristic are not perfectly horizontal straight lines. At each value of V BE , the corresponding ic;-vCE characteristic curve can be measured point-by-point by varying the dc source connected between collector and emitter and measuring the corresponding collector current. The result is the family of iC-VCE characteristic curves shown in Fig. We observe that the characteristic curves, though still straight lines, have finite slope. In fact, when extrapolated, the characteristic lines meet at a point on the negative VCE axis, at VCE = -VA, it is called early voltage. vBE is a given value, vCE I^ __ width of depletion region I^ __ base width decrease ___ Is I^ __Ic I^When operated in the active region, dependence of the collector current on the voltage exists, ic versus vCE characteristic are not perfectly horizontal straight lines. At each value of V BE , the corresponding ic;-vCE characteristic curve can be measured point-by-point by varying the dc source connected between collector and emitter and measuring the corresponding collector current. The result is the family of iC-VCE characteristic curves shown in Fig. We observe that the characteristic curves, though still straight lines, have finite slope. In fact, when extrapolated, the characteristic lines meet at a point on the negative VCE axis, at VCE = -VA, it is called early voltage. vBE is a given value, vCE I^ __ width of depletion region I^ __ base width decrease ___ Is I^ __Ic I^

    34. 2012/1/28 34 J.P. Chen The Early Effect The nonzero slope of the ic-VCE straight lines indicates that the output resistance looking into the collector is not infinite. Rather, it is finite and defined by the equation. It is rarely necessary to include the dependence of ic on VCE in dc bias design and analysis. However, the finite output resistance ro can have a significant effect on the gain of transistor amplifiers, as will be seen in later sections and chapters. The nonzero slope of the ic-VCE straight lines indicates that the output resistance looking into the collector is not infinite. Rather, it is finite and defined by the equation. It is rarely necessary to include the dependence of ic on VCE in dc bias design and analysis. However, the finite output resistance ro can have a significant effect on the gain of transistor amplifiers, as will be seen in later sections and chapters.

    35. 2012/1/28 35 J.P. Chen Homework: 4.1 ; 4.19; 4.20; 4.24

    36. SJTU J. Chen 36 2012/1/28 Part 2 DC analysis of transistor circuits;Small Signal Operation and Models

    37. 2012/1/28 37 J.P. Chen Large-signal Equivalent Circuit Models Analysis Steps Examples Analysis of Transistor Circuit at DC

    38. 2012/1/28 38 J.P. Chen Equivalent large signal Circuit Models

    39. 2012/1/28 39 J.P. Chen Equivalent large signal Circuit Models

    40. 2012/1/28 40 J.P. Chen Equivalent large signal Circuit Models

    41. 2012/1/28 41 J.P. Chen Using simple constant-voltage drop model, assuming irrespective of the exact value of currents. Assuming the device operates at the active region, we can apply the relationship between IB, IC, and IE, to determine the voltage VCE or VCB. Check the value of VCE or VCB, if VBE>0.7V and VC>VB, the assumption is correct. VBE>0.7V and VC<VB, the assumption is incorrect. It means the BJT is operating in saturation region. Thus we shall assume VCE=VCE(sat) (0.2~0.3V) to obtain IC. Here the common emitter current gain is defined as ?forced=IC/IB, we will find ?forced< ?. DC Analysis Steps

    42. 2012/1/28 42 J.P. Chen Example

    43. 2012/1/28 43 J.P. Chen Example

    44. 2012/1/28 44 J.P. Chen Example

    45. 2012/1/28 45 J.P. Chen Example

    46. 2012/1/28 46 J.P. Chen Example

    47. 2012/1/28 47 J.P. Chen Example

    48. 2012/1/28 48 J.P. Chen Conceptual Circuits Small-signal equivalent circuit models Application of the small-signal equivalent circuit models Augmenting the hybrid p model. The Transistor as an Amplifier

    49. 2012/1/28 49 J.P. Chen The Concept of Signal Amplification

    50. 2012/1/28 50 J.P. Chen Performance parameters of amplifier

    51. 2012/1/28 51 J.P. Chen Performance parameters of amplifier

    52. 2012/1/28 52 J.P. Chen Performance parameters of amplifier

    53. 2012/1/28 53 J.P. Chen Performance parameters of amplifier

    54. 2012/1/28 54 J.P. Chen Conceptual Circuit

    55. 2012/1/28 55 J.P. Chen Conceptual Circuit

    56. 2012/1/28 56 J.P. Chen Transconductance Input resistance at base Input resistance at emitter Hybrid p and T model Small-Signal Circuit Models

    57. 2012/1/28 57 J.P. Chen Small-Signal Model of a Diode

    58. 2012/1/28 58 J.P. Chen The Hybrid-? Model

    59. 2012/1/28 59 J.P. Chen The T Model

    60. 2012/1/28 60 J.P. Chen Expression Physical meaning gm is the slope of the iC–vBE curve at the bias point Q. At room temperature, Transconductance

    61. 2012/1/28 61 J.P. Chen Input resistance at base Input resistance at emitter Relationship between these two resistances Input Resistance at Base and Emitter

    62. 2012/1/28 62 J.P. Chen The analysis process Determine the dc operating point of the BJT, in particular Ic Calculate the values of the small-signal model parameters: Give the AC circuit: eliminate the dc source by replacing each dc voltage source with a short circuit and each dc current source with an open circuit Replace the BJT with one of its small-signal equivalent circuit models Analyze the resulting circuit to determine the required quantities, e.g. voltage gain, input resistance and output resistance

    63. 2012/1/28 63 J.P. Chen Augmenting the Hybrid-? Model

    64. 2012/1/28 64 J.P. Chen 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. Models for pnp Type

    65. 2012/1/28 65 J.P. Chen Summary of model parameters

    66. 2012/1/28 66 J.P. Chen Homework 4.36; 4.54; 4.57

    67. 2012/1/28 67 J.P. Chen Part 1. Physical Operation and Current-Voltage Characteristics. Part 2. DC analysis of transistor circuits;Small Signal Operation and Models Part 3. The basic single-stage BJT amplifier and configurations Content

    68. 2012/1/28 68 J.P. Chen 4.9 Graphical Analysis (DC)

    69. 2012/1/28 69 J.P. Chen Graphical Analysis (DC)

    70. 2012/1/28 70 J.P. Chen Small Signal Analysis

    71. 2012/1/28 71 J.P. Chen Effect of Bias-Point Location on Allowable Signal Swing

    72. 2012/1/28 72 J.P. Chen Biasing with voltage Classical discrete circuit bias arrangement Single power supply Two-power-supply With feedback resistor Biasing with current source 4.10 Biasing in BJT Amplifier Circuit

    73. 2012/1/28 73 J.P. Chen Why should we locate the proper bias?

    74. 2012/1/28 74 J.P. Chen e.g Q-point change by temperature

    75. 2012/1/28 75 J.P. Chen Classical Discrete Circuit Bias Arrangement

    76. 2012/1/28 76 J.P. Chen Classical Discrete Circuit Bias Arrangement

    77. 2012/1/28 77 J.P. Chen Classical Biasing for BJTs Using a Single Power Supply

    78. 2012/1/28 78 J.P. Chen Classical Biasing for BJTs Using a Single Power Supply Two constraints Rules of thumb

    79. 2012/1/28 79 J.P. Chen Resistor RB can be eliminated in common base configuration. Resistor RB is needed only if the signal is to be capacitively coupled to the base. Two constraints should apply. Two-Power-Supply Version

    80. 2012/1/28 80 J.P. Chen Biasing with Feedback Resistor

    81. 2012/1/28 81 J.P. Chen Biasing Using Current Source

    82. 2012/1/28 82 J.P. Chen Determine the DC operating point of BJT and in particular the DC collector current IC(ICQ). Calculate the values of the small-signal model parameters, such as gm=IC/VT, r?=?/gm=VT/IB, re=?/gm=VT/IE. Draw ac circuit path. Replace the BJT with one of its small-signal models. The model selected may be more convenient than the others in circuits analysis. Determine the required quantities. Application of the Small-Signal Models

    83. 2012/1/28 83 J.P. Chen Characteristic parameters Basic structure Configuration Common-Emitter amplifier Emitter directly connects to ground Emitter connects to ground by resistor RE Common-base amplifier Common-collector amplifier(emitter follower) 4.11 Basic Single-Stage BJT Amplifier

    84. 2012/1/28 84 J.P. Chen Characteristic Parameters of Amplifier

    85. 2012/1/28 85 J.P. Chen Input resistance with no load Input resistance Open-circuit voltage gain Voltage gain Definitions

    86. 2012/1/28 86 J.P. Chen Short-circuit current gain Current gain Short-circuit transconductance Definitions

    87. 2012/1/28 87 J.P. Chen Open-circuit overall voltage gain Overall voltage gain Definitions

    88. 2012/1/28 88 J.P. Chen Definitions

    89. 2012/1/28 89 J.P. Chen Definitions

    90. 2012/1/28 90 J.P. Chen Voltage divided coefficient Relationships

    91. 2012/1/28 91 J.P. Chen Basic Structure

    92. 2012/1/28 92 J.P. Chen Common-Emitter Amplifier

    93. 2012/1/28 93 J.P. Chen DC & AC circuits DC circuit AC circuit

    94. 2012/1/28 94 J.P. Chen e.g. 100 ?F v.s. 1 k? @ 40 Hz

    95. 2012/1/28 95 J.P. Chen Common-Emitter Amplifier

    96. 2012/1/28 96 J.P. Chen Input resistance Voltage gain Overall voltage gain Output resistance Short-circuit current gain Characteristics of CE Amplifier

    97. 2012/1/28 97 J.P. Chen Large voltage gain Inverting amplifier Large current gain Input resistance is relatively low. Output resistance is relatively high. Frequency response is rather poor. Summary of CE amplifier

    98. 2012/1/28 98 J.P. Chen The Common-Emitter Amplifier with a Resistance in the Emitter

    99. 2012/1/28 99 J.P. Chen The Common-Emitter Amplifier with a Resistance in the Emitter

    100. 2012/1/28 100 J.P. Chen Input resistance Voltage gain Overall voltage gain Output resistance Short-circuit current gain Characteristics of the CE Amplifier with a Resistance in the Emitter

    101. 2012/1/28 101 J.P. Chen The input resistance Rin is increased by the factor (1+gmRe) The voltage gain from base to collector is reduced by the factor (1+gmRe). For the same nonlinear distortion, the input signal vi can be increased by the factor (1+gmRe). The overall voltage gain is less dependent on the value of ß. Summary of CE Amplifier with RE

    102. 2012/1/28 102 J.P. Chen The reduction in gain is the price for obtaining the other performance improvement. Resistor RE introduces the negative feedback into the amplifier. The high frequency response is significant improved. Summary of CE Amplifier with RE

    103. 2012/1/28 103 J.P. Chen Common-Base Amplifier

    104. 2012/1/28 104 J.P. Chen Common-Base Amplifier

    105. 2012/1/28 105 J.P. Chen Characteristics of CB Amplifier Input resistance Voltage gain Overall voltage gain Output resistance Short-circuit current gain

    106. 2012/1/28 106 J.P. Chen 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 Summary of the CB Amplifier

    107. 2012/1/28 107 J.P. Chen The Common-Collector Amplifier or Emitter-Follower

    108. 2012/1/28 108 J.P. Chen The Common-Collector Amplifier or Emitter-Follower

    109. 2012/1/28 109 J.P. Chen The Common-Collector Amplifier or Emitter-Follower

    110. 2012/1/28 110 J.P. Chen Input resistance Voltage gain Overall voltage gain Output resistance Short-circuit current gain Characteristics of CC Amplifier

    111. 2012/1/28 111 J.P. Chen 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 Summary for CC Amplifier or Emitter-Follower

    112. 2012/1/28 112 J.P. Chen The CE configuration is best suited for realizing the amplifier gain. Including RE provides performance improvements at the expense of gain reduction. The CB configuration has superior high-frequency response. The emitter follower can be used as a voltage buffer and exists in output stage of a multistage amplifier. Summary and Comparisons

    113. 2012/1/28 113 J.P. Chen Summary and Comparisons

    114. 2012/1/28 114 J.P. Chen 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 Internal Capacitances of the BJT and High Frequency Model

    115. 2012/1/28 115 J.P. Chen 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

    116. 2012/1/28 116 J.P. Chen The Base-Emitter Junction Capacitance The collector-base junction capacitance Junction Capacitances

    117. 2012/1/28 117 J.P. Chen Two capacitances Cp and Cµ , where One resistance rx . Accurate value is obtained form high frequency measurement. The High-Frequency Hybrid-? Model

    118. 2012/1/28 118 J.P. Chen Circuit for deriving an expression for According to the definition, output port is short circuit The Cutoff and Unity-Gain Frequency

    119. 2012/1/28 119 J.P. Chen Expression of the short-circuit current transfer function Characteristic is similar to the one of first-order low-pass filter The Cutoff and Unity-Gain Frequency

    120. 2012/1/28 120 J.P. Chen The Cutoff and Unity-Gain Frequency

    121. 2012/1/28 121 J.P. Chen Homework 4.78, 4.84, 4.89, 4.95

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