1. Bipolar Junction Transistor (BJT)
Chapter 3
Bipolar Junction Transistor (BJT)
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2. 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
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3. Introduction Physical Structure Circuit Symbols for BJTs
Modes of Operation
Basic Characteristic
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4. Physical Structure A simplified structure of the npn transistor.
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5. Physical Structure
A dual of the npn is called pnp type. This is the simplified structure of the pnp transistor.
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6. Circuit Symbols for BJTs
The emitter is distinguished by the arrowhead.
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7. Modes of Operation Modes EBJ CBJ Application Cutoff Reverse
Switching application in digital circuits
Saturation
Forward
Active
Amplifier
Reverse active
Performance degradation
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8. 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
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9. Operation in the Active Mode
Current flow
Current equation
Graphical representation of transistor’s characteristics
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10. Current Flow
Current flow in an npn transistor biased to operate in the active mode.
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11. 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.
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12. Base Current Base current consists of two components.
Diffusion current
Recombination current
Recombination current is dominant.
The value of base current is very small.
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13. 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.
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14. Profiles of Minority-Carrier Concentrations
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15. Current Equation Collector current Base current Emitter current
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16. 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 10-12A to 10-18A.
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17. 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.
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18. 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.
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19. 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.
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20. 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.
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21. Output Curves for CB Configuration
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22. 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.
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23. Output Curves for CE Configuration
(a) Conceptual circuit for measuring the iC –vCE characteristics of the BJT.
(b) The iC –vCE characteristics of a practical BJT.
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24. The Early Effect
Curves in active region are more sloped than those in CB configuration.
Early voltage.
Effective base width and base width modulation.
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25. 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
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26. Analysis of Transistor Circuit at DC
Equivalent Circuit Models
Analysis Steps
Examples
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27. 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.
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28. DC Analysis Steps
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
VC>VB (or VCE>0.2V), the assumption is correct.
VC<VB (or VCE<0.2V), the assumption is incorrect. It means the BJT is operating in saturation region. Thus we shall assume VCE=VCE(sat) to obtain IC. Here the common emitter current gain is defined as forced=IC/IB, we will find forced< .
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29. 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.
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30. 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.
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31. 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.
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32. 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.
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33. Conceptual Circuit
(a) Conceptual circuit to illustrate the operation of the transistor as an amplifier.
(b) The circuit of (a) with the signal source vbe eliminated for dc (bias) analysis.
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34. Conceptual Circuit(cont’d)
With the dc sources (VBE and VCC) 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.
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35. Small-Signal Circuit Models
Transconductance
Input resistance at base
Input resistance at emitter
Hybrid π and T model
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36. Transconductance Expression Physical meaning gm is the slope of the
iC–vBE curve at the bias point Q.
At room temperature,
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37. Input Resistance at Base and Emitter
Input resistance at emitter
Relationship between these two resistances
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38. 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).
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39. The T Model
These models explicitly show the emitter resistance re rather than the base resistance rp featured in the hybrid-p model.
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40. Augmenting the Hybrid- Model
Expression for the output resistance.
Output resistance represents the Early Effect(or base width modulation)
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41. 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.
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42. Graphical Analysis
Graphical construction for the determination of the dc base current in the circuit.
Load line intersects with the input characteristic curve.
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43. Graphical Analysis(cont’d)
Graphical construction for determining the dc collector current IC and the collector-to-emitter voltage VCE in the circuit.
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44. Small Signal Analysis
Graphical determination of the signal components vbe, ib, ic, and vce when a signal component vi is superimposed on the dc voltage VBB
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45. Effect of Bias-Point Location on Allowable Signal Swing
Load-line A results in bias point QA with a corresponding VCE which is too close to VCC and thus limits the positive swing of vCE.
At the other extreme, load-line B results in an operating point too close to the saturation region, thus limiting the negative swing of vCE.
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46. 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
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47. Classical Discrete Circuit Bias Arrangement
by fixing VBE by fixing IB.
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48. Classical Discrete Circuit Bias Arrangement
Both result in wide variations in IC and hence in VCE and therefore are considered to be “bad.”
Neither scheme is recommended.
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49. 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 RE
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50. Classical Biasing for BJTs Using a Single Power Supply
Two constraints
Rules of thumb
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51. Two-Power-Supply Version
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.
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52. Biasing with Feedback Resistor
Resistor RB provides negative feedback.
IE is insensitive to β provided that
The value of RB determines the allowable signal swing at the collector.
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53. Biasing Using Current Source
Q1 and Q2 are required to be identical and have high β.
Short circuit between Q1’s base and collector terminals.
Current source isn’t ideal due to finite output resistor of Q2
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54. Application of the Small-Signal Models
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.
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55. Basic Single-Stage BJT Amplifier
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)
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56. Characteristic Parameters of Amplifier
This is the two-port network of amplifier.
Voltage signal source.
Output signal is obtained from the load resistor.
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57. Definitions Input resistance with no load Input resistance
Open-circuit voltage gain
Voltage gain
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58. Definitions(cont’d) Short-circuit current gain Current gain
Short-circuit transconductance
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59. Definitions(cont’d) Open-circuit overall voltage gain
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60. Definitions(cont’d) Output resistance of amplifier proper
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61. Definitions(cont’d) Voltage amplifier Voltage amplifier
Transconductance amplifier
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62. Relationships
Voltage divided coefficient
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63. Basic Structure
Basic structure of the circuit used to realize single-stage, discrete-circuit BJT amplifier configurations.
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64. Common-Emitter Amplifier
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65. Common-Emitter Amplifier
Equivalent circuit obtained by replacing the transistor with its hybrid-p model.
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66. Characteristics of CE Amplifier
Input resistance
Voltage gain
Overall voltage gain
Output resistance
Short-circuit current gain
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67. 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.
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68. The Common-Emitter Amplifier with a Resistance in the Emitter
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69. The Common-Emitter Amplifier with a Resistance in the Emitter
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70. Characteristics of the CE Amplifier with a Resistance in the Emitter
Input resistance
Voltage gain
Overall voltage gain
Output resistance
Short-circuit current gain
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71. Summary of CE Amplifier with RE
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 β.
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72. Summary of CE Amplifier with RE
The reduction in gain is the price for obtaining the other performance improvements.
Resistor RE introduces the negative feedback into the amplifier.
The high frequency response is significant improved.
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73. Common-Base Amplifier
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74. Common-Base Amplifier
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75. Characteristics of CB Amplifier
Input resistance
Voltage gain
Overall voltage gain
Output resistance
Short-circuit current gain
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76. 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
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77. The Common-Collector Amplifier or Emitter-Follower
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78. The Common-Collector Amplifier or Emitter-Follower
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79. The Common-Collector Amplifier or Emitter-Follower
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80. Characteristics of CC Amplifier
Input resistance
Voltage gain
Overall voltage gain
Output resistance
Short-circuit current gain
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81. 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
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82. Summary and Comparisons
The CE configuration is the best suited for realizing the amplifier gain.
Including RE 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.
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83. Internal Capacitances of the BJT and High Frequency Model
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
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84. 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
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85. Junction Capacitances
The Base-Emitter Junction Capacitance
The collector-base junction capacitance
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86. The High-Frequency Hybrid- Model
Two capacitances Cπ and Cμ , where
One resistance rx . Accurate value is obtained form high frequency measurement.
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87. The Cutoff and Unity-Gain Frequency
Circuit for deriving an expression for
According to the definition, output port is short circuit
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88. 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
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89. The Cutoff and Unity-Gain Frequency (cont’d)
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