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Chapter 4 – Bipolar Junction Transistors (BJTs)

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Chapter 4 – Bipolar Junction Transistors (BJTs). Introduction. http://engr.calvin.edu/PRibeiro_WEBPAGE/courses/engr311/311_frames.html. Physical Structure and Modes of Operation. A simplified structure of the npn transistor. Physical Structure and Modes of Operation.

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slide1
Chapter 4 – Bipolar Junction Transistors (BJTs)

Introduction

http://engr.calvin.edu/PRibeiro_WEBPAGE/courses/engr311/311_frames.html

slide2
Physical Structure and Modes of Operation

A simplified structure of the npn transistor.

slide3
Physical Structure and Modes of Operation

A simplified structure of the pnp transistor.

slide4
Physical Structure and Modes of Operation

Mode EBJ CBJ

Active Forward Reverse

Cutoff Reverse Reverse

Saturation Forward Forward

slide5
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.)

slide6
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.

slide7
Operation of The npn Transistor Active Mode

The Collector Current

The Base Current

Physical Structure and Modes of Operation

slide8
Equivalent Circuit Models

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

slide9
The Constant n
    • The Collector-Base Reverse Current
    • The Structure of Actual Transistors
slide10
The pnp Transistor

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

slide11
The pnp Transistor

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

slide21
The Graphical Representation of the Transistor Characteristics

Temperature Effect (10 to 120 C)

slide22
Dependence of ic on the Collector Voltage

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

slide24
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-vCEcharacteristics of a practical BJT.

slide33
Monte Carlo Analysis – Using PSpice

Probe Output

Ic(Q), Ib(Q), Vce

slide34
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

slide35
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.

slide36
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).

slide37
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 rerather than the base resistance r featured in the hybrid- model.

slide39
Fig.4.30 Example 4.11: (a) circuit; (b) dc analysis; (c) small-signal model; (d) small-signal analysis performed directly on the circuit.
slide41
Fig.4.35 Graphical construction for the determination of the dc base current in the circuit of Fig.4.34.
slide42
Fig. 4.36 Graphical construction for determining the dc collector current IC and the collector-to-emmiter voltage VCE in the circuit of Fig. 4.34.
slide43
Fig.4.37 Graphical determination of the signal components vbe, ib, ic, and vce when a signal component viis superimposed on the dc voltage VBB(see Fig.4.34).
slide44
Fig.4.38 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.
slide45
Fig.4.44 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.
slide46
Fig.4.45 The common-base amplifier. (a) Circuit. (b) Equivalent circuit obtained by replacing the BJT with its T model.
slide47
Fig.4.46 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.
slide48
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

slide50
A General Large-Signal Model For The BJT:

The Ebers-Moll Model – Terminal Currents

slide51
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

slide52
A General Large-Signal Model For The BJT:

The Ebers-Moll Model – Normal Saturation

slide53
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

slide54
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.

slide55
A General Large-Signal Model For The BJT:

The Ebers-Moll Model – Example

slide56
A General Large-Signal Model For The BJT:

The Ebers-Moll Model – Example

slide57
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 iTis defined by Eq. (4.117).

slide58
Basic BJT Digital Logic Inverter.

vi high (close to power supply) - vo low

vi low vo high

Basic BJT digital logic inverter.

slide59
Basic BJT Digital Logic Inverter.

Sketch of the voltage transfer characteristic of the inverter circuit of Fig. 4.60 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.

slide60
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.

slide61
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.

slide62
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.

slide63
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.

slide64
Complete Static Characteristics, Internal Impedances,

and Second-Order Effects – Common-Emitter

An expanded view of the common-emitter characteristics in the saturation region.

slide72
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=TO18 88-09-07 bam creation

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