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Chapter 4 – Bipolar Junction Transistors (BJTs) Introduction

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1 Chapter 4 – Bipolar Junction Transistors (BJTs) Introduction

2 A simplified structure of the npn transistor. Physical Structure and Modes of Operation

3 A simplified structure of the pnp transistor. Physical Structure and Modes of Operation

4 ModeEBJCBJ ActiveForwardReverse CutoffReverseReverse SaturationForwardForward

5 Current flow in an npn transistor biased to operate in the active mode, (Reverse current components due to drift of thermally generated minority carriers are not shown.) Operation of The npn Transistor Active Mode

6 Profiles of minority-carrier concentrations in the base and in the emitter of an npn transistor operating in the active mode; v BE  0 and v CB  0. Operation of The npn Transistor Active Mode

7 The Collector Current The Base Current Physical Structure and Modes of Operation Operation of The npn Transistor Active Mode

8 Large-signal equivalent-circuit models of the npn BJT operating in the active mode. Equivalent Circuit Models

9 The Constant n The Collector-Base Reverse Current The Structure of Actual Transistors

10 Current flow in an pnp transistor biased to operate in the active mode. The pnp Transistor

11 Two large-signal models for the pnp transistor operating in the active mode. The pnp Transistor

12 Circuit Symbols and Conventions C B E C B E

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14 Example 4.1 E B C

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18 Exercise 4.8

19 Exercise 4.9

20 The Graphical Representation of the Transistor Characteristics

21 Temperature Effect (10 to 120 C)

22 The i C -v CB characteristics for an npn transistor in the active mode. Dependence of ic on the Collector Voltage

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24 (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. Dependence of ic on the Collector Voltage – Early Effect VA – 50 to 100V

25 Dependence of ic on the Collector Voltage – Early Effect

26 Nested DC Sweeps

27 Example

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30 Monte Carlo Analysis – Using PSpice

31 Monte Carlo Analysis – Using PSpice

32 Monte Carlo Analysis – Using PSpice

33 Probe Output Ic(Q), Ib(Q), Vce Monte Carlo Analysis – Using PSpice

34 (a) Conceptual circuit to illustrate the operation of the transistor of an amplifier. (b) The circuit of (a) with the signal source v be eliminated for dc (bias) analysis. The Transistor As An Amplifier The Collector Current and The Transconductance The Base Current and the Input Resistance at the Base The Emitter Current and the Input Resistance at the Emitter

35 Linear operation of the transistor under the small-signal condition: A small signal v be with a triangular waveform is superimpose din the dc voltage V BE. It gives rise to a collector signal current i c, also of triangular waveform, superimposed on the dc current I C. I c = g m v be, where g m is the slope of the i c - v BE curve at the bias point Q. The Transistor As An Amplifier

36 Two slightly different versions of the simplified hybrid-  model for the small-signal operation of the BJT. The equivalent circuit in (a) represents the BJT as a voltage-controlled current source ( a transconductance amplifier) and that in (b) represents the BJT as a current-controlled current source (a current amplifier). Small-Signal Equivalent Circuit Models

37 Two slightly different versions of what is known as the T model of the BJT. The circuit in (a) is a voltage-controlled current source representation and that in (b) is a current-controlled current source representation. These models explicitly show the emitter resistance r e rather than the base resistance r  featured in the hybrid-  model. Small-Signal Equivalent Circuit Models

38 Signal waveforms in the circuit of Fig

39 Fig Example 4.11: (a) circuit; (b) dc analysis; (c) small-signal model; (d) small-signal analysis performed directly on the circuit.

40 Fig Circuit whose operation is to be analyzed graphically.

41 Fig Graphical construction for the determination of the dc base current in the circuit of Fig

42 Fig Graphical construction for determining the dc collector current I C and the collector-to-emmiter voltage V CE in the circuit of Fig

43 Fig 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 (see Fig. 4.34).

44 Fig Effect of bias-point location on allowable signal swing: 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. 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.

45 Fig The common-emitter amplifier with a resistance R e in the emitter. (a) Circuit. (b) Equivalent circuit with the BJT replaced with its T model (c) The circuit in (b) with r o eliminated.

46 Fig The common-base amplifier. (a) Circuit. (b) Equivalent circuit obtained by replacing the BJT with its T model.

47 Fig The common-collector or emitter-follower amplifier. (a) Circuit. (b) Equivalent circuit obtained by replacing the BJT with its T model. (c) The circuit in (b) redrawn to show that r o is in parallel with R L. (d) Circuit for determining R o.

48 An npn resistor and its Ebers-Moll (EM) model. ISC and ISE are the scale or saturation currents of diodes D E (EBJ) and D C (CBJ). More General – Describe Transistor in any mode of operation. Base for the Spice model. Low frequency only A General Large-Signal Model For The BJT: The Ebers-Moll Model ISC > ISE (2-50)

49 A General Large-Signal Model For The BJT: The Ebers-Moll Model

50 A General Large-Signal Model For The BJT: The Ebers-Moll Model – Terminal Currents

51 A General Large-Signal Model For The BJT: The Ebers-Moll Model – Forward Active Mode Since vBC is negative and its magnitude Is usually much greater than VT the Previous equations can be approximated as

52 A General Large-Signal Model For The BJT: The Ebers-Moll Model – Normal Saturation

53 A General Large-Signal Model For The BJT: The Ebers-Moll Model – Reverse Mode I1 I2IB Note that the currents indicated have positive values. Thus, since ic = -I2 and iE = -I1, both iC and IE will be negative. Since the roles of the emitter and collector are interchanged, the transistor in the circuit will operate in the active mode (called the reverse active mode) when the emitter-base junction is reverse-biased. In such a case I1 = beta_R. IB This circuit will saturate (reverse saturation mode) when the emitter-base junction becomes forward-biased. I1/IB < beta_R

54 A General Large-Signal Model For The BJT: The Ebers-Moll Model – Reverse Saturation We can use the EM equations to find the expression of VECSat From this expression, it can be seen that the minimum VECSat is obtained when I1 = 0. This minimum is very close to zero. The disadvantage of the reverse saturation mode is a relatively long turnoff time.

55 A General Large-Signal Model For The BJT: The Ebers-Moll Model – Example

56 A General Large-Signal Model For The BJT: The Ebers-Moll Model – Example

57 The transport model of the npn BJT. This model is exactly equivalent to the Ebers-Moll model. Note that the saturation currents of the diodes are given in parentheses and i T is defined by Eq. (4.117). A General Large-Signal Model For The BJT: The Ebers-Moll Model – Transport Model npn BJT

58 Basic BJT digital logic inverter. Basic BJT Digital Logic Inverter. vi high (close to power supply) - vo low vi low vo high

59 Sketch of the voltage transfer characteristic of the inverter circuit of Fig for the case R B = 10 k , R C = 1 k ,  = 50, and V CC = 5V. For the calculation of the coordinates of X and Y refer to the text. Basic BJT Digital Logic Inverter.

60 (a) The minority-carrier concentration in the base of a saturated transistor is represented by line (c). (b) The minority-carrier charge stored in the base can de divided into two components: That in blue produces the gradient that gives rise to the diffusion current across the base, and that in gray results in driving the transistor deeper into saturation. The Voltage Transfer Characteristics

61 The i c -v cb or common-base characteristics of an npn transistor. Note that in the active region there is a slight dependence of i C on the value of v CB. The result is a finite output resistance that decreases as the current level in the device is increased. Complete Static Characteristics, Internal Impedances, and Second-Order Effects – Common Base Avalanche Saturation Slope

62 The hybrid-  model, including the resistance r , which models the effect of v c on i b. Complete Static Characteristics, Internal Impedances, and Second-Order Effects – Common Base

63 Common-emitter characteristics. Note that the horizontal scale is expanded around the origin to show the saturation region in some detail. Complete Static Characteristics, Internal Impedances, and Second-Order Effects – Common-Emitter

64 An expanded view of the common-emitter characteristics in the saturation region. Complete Static Characteristics, Internal Impedances, and Second-Order Effects – Common-Emitter

65 The Transistor Beta

66 Transistor Breakdown

67 Internal Capacitances of a BJT

68 The Cut-Off Frequency

69 The Spice BJT Model and Simulation Examples

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72 .model Q2N2222-X NPN( Is=14.34f Xti=3 Eg=1.11 Vaf=74.03 Bf=200 Ne=1.307 Ise=14.34f Ikf=.2847 Xtb=1.5 Br=6.092 Nc=2 Isc=0 Ikr=0 Rc=1 Cjc=7.306p Mjc=.3416 Vjc=.75 Fc=.5 Cje=22.01p Mje=.377 Vje=.75 Tr=46.91n Tf=411.1p Itf=.6 Vtf=1.7 Xtf=3 Rb=10) *Nationalpid=19 case=TO bamcreation The Spice BJT Model and Simulation Examples

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74 BJT Modeling - Idealized Cross Section of NPN BJT

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77 The Spice BJT Model and Simulation Examples


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