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**Chapter 4 – Bipolar Junction Transistors (BJTs)**

Introduction

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**Physical Structure and Modes of Operation**

A simplified structure of the npn transistor.

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**Physical Structure and Modes of Operation**

A simplified structure of the pnp transistor.

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**Physical Structure and Modes of Operation**

Mode EBJ CBJ Active Forward Reverse Cutoff Reverse Reverse Saturation Forward Forward

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**Operation of The npn Transistor Active Mode**

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.)

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**Operation of The npn Transistor Active Mode**

Profiles of minority-carrier concentrations in the base and in the emitter of an npn transistor operating in the active mode; vBE 0 and vCB 0.

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**Operation of The npn Transistor Active Mode**

The Collector Current The Base Current Physical Structure and Modes of Operation

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**Equivalent Circuit Models**

Large-signal equivalent-circuit models of the npn BJT operating in the active mode.

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

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

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

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**Circuit Symbols and Conventions**

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**Circuit Symbols and Conventions**

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Example 4.1 C B E

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Example 4.1

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Example 4.1

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Exercise 4.8

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Exercise 4.9

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**The Graphical Representation of the Transistor Characteristics**

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**The Graphical Representation of the Transistor Characteristics**

Temperature Effect (10 to 120 C)

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**Dependence of ic on the Collector Voltage**

The iC-vCB characteristics for an npn transistor in the active mode.

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**Dependence of ic on the Collector Voltage**

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**Dependence of ic on the Collector Voltage – Early Effect**

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

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Nested DC Sweeps

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Example

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Example

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Example

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**Monte Carlo Analysis – Using PSpice**

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**Monte Carlo Analysis – Using PSpice**

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**Monte Carlo Analysis – Using PSpice**

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**Monte Carlo Analysis – Using PSpice**

Probe Output Ic(Q), Ib(Q), Vce

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**The Transistor As An Amplifier**

(a) Conceptual circuit to illustrate the operation of the transistor of an amplifier. (b) The circuit of (a) with the signal source vbe eliminated for dc (bias) analysis. 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

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**The Transistor As An Amplifier**

Linear operation of the transistor under the small-signal condition: A small signal vbe with a triangular waveform is superimpose din the dc voltage VBE. It gives rise to a collector signal current ic, also of triangular waveform, superimposed on the dc current IC. Ic = gm vbe, where gm is the slope of the ic - vBE curve at the bias point Q.

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**Small-Signal Equivalent Circuit Models**

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).

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**Small-Signal Equivalent Circuit Models**

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 re rather than the base resistance r featured in the hybrid- model.

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**Signal waveforms in the circuit of Fig. 4.28.**

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

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**Fig. 4.34 Circuit whose operation is to be analyzed graphically.**

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

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Fig Graphical construction for determining the dc collector current IC and the collector-to-emmiter voltage VCE in the circuit of Fig

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Fig Graphical determination of the signal components vbe, ib, ic, and vce when a signal component vi is superimposed on the dc voltage VBB (see Fig. 4.34).

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Fig 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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Fig The common-emitter amplifier with a resistance Re in the emitter. (a) Circuit. (b) Equivalent circuit with the BJT replaced with its T model (c) The circuit in (b) with ro eliminated.

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**Fig. 4. 45 The common-base amplifier. (a) Circuit**

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

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**Fig. 4. 46 The common-collector or emitter-follower amplifier**

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 ro is in parallel with RL. (d) Circuit for determining Ro.

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

ISC > ISE (2-50) An npn resistor and its Ebers-Moll (EM) model. ISC and ISE are the scale or saturation currents of diodes DE (EBJ) and DC (CBJ). More General – Describe Transistor in any mode of operation. Base for the Spice model. Low frequency only

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**A General Large-Signal Model For The BJT:**

The Ebers-Moll Model

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**A General Large-Signal Model For The BJT:**

The Ebers-Moll Model – Terminal Currents

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**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

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**A General Large-Signal Model For The BJT:**

The Ebers-Moll Model – Normal Saturation

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**A General Large-Signal Model For The BJT: **

The Ebers-Moll Model – Reverse Mode 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 I1 IB I2

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**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 = This minimum is very close to zero. The disadvantage of the reverse saturation mode is a relatively long turnoff time.

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**A General Large-Signal Model For The BJT:**

The Ebers-Moll Model – Example

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**A General Large-Signal Model For The BJT:**

The Ebers-Moll Model – Example

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**A General Large-Signal Model For The BJT: **

The Ebers-Moll Model – Transport Model npn BJT 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 iT is defined by Eq. (4.117).

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**Basic BJT Digital Logic Inverter.**

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

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**Basic BJT Digital Logic Inverter.**

Sketch of the voltage transfer characteristic of the inverter circuit of Fig for the case RB = 10 k, RC = 1 k, = 50, and VCC = 5V. For the calculation of the coordinates of X and Y refer to the text.

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**The Voltage Transfer Characteristics**

(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.

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**Complete Static Characteristics, Internal Impedances, **

and Second-Order Effects – Common Base Avalanche Saturation Slope The ic-vcb or common-base characteristics of an npn transistor. Note that in the active region there is a slight dependence of iC on the value of vCB. The result is a finite output resistance that decreases as the current level in the device is increased.

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**Complete Static Characteristics, Internal Impedances, **

and Second-Order Effects – Common Base The hybrid- model, including the resistance r, which models the effect of vc on ib.

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**Complete Static Characteristics, Internal Impedances, **

and Second-Order Effects – Common-Emitter Common-emitter characteristics. Note that the horizontal scale is expanded around the origin to show the saturation region in some detail.

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**Complete Static Characteristics, Internal Impedances, **

and Second-Order Effects – Common-Emitter An expanded view of the common-emitter characteristics in the saturation region.

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The Transistor Beta

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Transistor Breakdown

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**Internal Capacitances of a BJT**

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The Cut-Off Frequency

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**The Spice BJT Model and Simulation Examples**

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**The Spice BJT Model and Simulation Examples**

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**The Spice BJT Model and Simulation Examples**

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**The Spice BJT Model and Simulation Examples**

.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) *National pid=19 case=TO bam creation

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**The Spice BJT Model and Simulation Examples**

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**BJT Modeling - Idealized Cross Section of NPN BJT**

BJT Modeling - Idealized Cross Section of NPN BJT

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**The Spice BJT Model and Simulation Examples**

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