Bipolar Junction Transistors (BJTs)dicle.edu.tr/akademikpersonel/sozerdem/EEZ204/BJTs_ch3.pdf5 Microelectronic Circuits - Fifth Edition Sedra/Smith Copyright 2004 by Oxford University

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Copyright 2004 by Oxford University Press, Inc.

ELZ 303 - Elektronik II

Dr. Mehmet Siraç Özerdem

Elektrik Elektronik Mühendisliğ Bölümü

Dicle Üniversitesi

Microelectronic Circuits – Fourth Edition

Adel S. Sedra, Kenneth C. Smith, 1998 Oxford University Press

Bipolar Junction

Transistors (BJTs)


A simplified structure of the

npn transistor.

Physical Structure

A simplified structure of the

pnp transistor.


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Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith

Cross-section of an npn BJT.

Dr. MS Ozerdem

Physical Structure


Mode EBJ CBJ Application(s)

Cutoff Reverse Reverse Switching (Logic circuits)

Active Forward Reverse Amplifier

Saturation Forward Forward Switching (Logic circuits)

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Modes of Operation

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Reverse current components due to drift of thermally generated

minority carriers are not shown.

The npn TransistorDr. MS Ozerdem

Current flow in an npn transistor biased to operate in

the active mode.


The Collector Current ( iC )

The Base Current ( iB )

The Emitter Current ( iE )

Current flow


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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.

Dr. MS Ozerdem

Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith 8

AE : Cross section area of BJT

q : Magnitude of electron charge

Dn : Electron diffusivity in the base

(- sign ) : Neg. slope results in a negative current in accross the

base (in the neg. direction of x)

W : Effective width of the base

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IS : Saturation current

The Collector Current

ni : intrinsic carrier density

NA : Doping concentration of the base

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The Base Current

ND : Doping concentration of the emitter

Dp : Hole diffusivity in the base

Lp : Hole diffusion length in the emitter

iB2 : Positive charge (Qn) every τb seconds

Qn : Minority-carrier charge in τb seconds in the

base recombinations with holes

τb : Average time for a minority electron to

recombine with a majority hole in the base

denoted τb (minority carrier lifetime)

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The Base Current

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The Base Current

β : Common emitter current gain

modern npn range 100 ≤ β ≤ 200

Special β > 1000

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The Emitter Current

α : Common base current gain

α is less than but very close to unity

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Large-signal equivalent-circuit models of the npn BJT operating

in the forward active mode.

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Equivalent Circuit Models

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The pnp Transistor

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


Large-signal model for the pnp transistor operating in the active mode.

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Circuit symbols for BJTs

Voltage polarities and

current flow in

transistors biased in the

active mode.


Example


= 100

VBE = 0.7V at IC=1mA

Design the circuit so that

IC = 2mA

VC = 5V

RC = ?

RE = ?

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Exercise

= 50

VE = - 0.7V

IE = ?

IB = ?

IC = ?

VC = ?

Results

IE = 0.93mA IB = 18.2μA

IC = 0.91mA VC = 5.45V


Example

VB = 1.0 V

VE = 1.7 V

α = ?

= ?

VC = ?


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The iC –vBE characteristic for an npn

transistor.

Effect of temperature on the iC–vBE

characteristic. At a constant emitter

current (broken line), vBE changes by

–2 mV/ C.

Dr. MS Ozerdem

Graphical Representation of Transistor Characteristics


Dr. MS Ozerdem

The iC–vCB characteristics of an npn transistor

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(a) Conceptual circuit for measuring the iC –vCE characteristics of the BJT.

(b) The iC –vCE characteristics of a practical BJT.

Dependence of iC on the Collector Voltage -

The Early Effect

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W → IS

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Large-signal equivalent-circuit models of an npn BJT operating in the

active mode in the common-emitter configuration.

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npn pnp

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Large-signal equivalent-circuit models of an BJT operating in the

active mode in the common-emitter configuration.

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Analysis of Transistor Circuits at DC

=100

VBE = 0.7 V

IE = ?

IB = ?

IC = ?

VC = ?

VE = ?

Example 1



Example 2

=100

VBE = 0.7 V

IE = ?

IB = ?

IC = ?

VC = ?

VE = ?

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Example 3

=100

VBE = 0.7 V

IE = ?

IB = ?

IC = ?

VC = ?

VE = ?



Example 4

=100

VBE = 0.7 V

IE = ?

IB = ?

IC = ?

VC = ?

VE = ?


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(a) circuit;

(b) analysis with the steps indicated by the circled numbers.

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Example 5


(a) circuit;

(b) analysis with steps numbered.

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Example 6

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Example 7


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Conceptual circuit to illustrate

the operation of the transistor as

an amplifier.

The Transistor as an Amplifier

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(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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The collector current

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Linear operation of the

transistor under the small-

signal condition: A small signal

vbe with a triangular waveform

is superimposed on the dc

voltage VBE. It gives rise to a

collector signal current ic, also

of triangular waveform,

superimposed on the dc current

IC. Here, ic = gmvbe, where gm is

the slope of the iC–vBE curve at

the bias point Q.

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The base current

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The emitter current

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(a) 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.

Dr. MS Ozerdem


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

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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.

Dr. MS Ozerdem


(a) circuit;

(b) dc analysis;

(c) small-signal model.

Example

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Dr. MS Ozerdem


(a) circuit;

(b) dc analysis;

(c) small-signal model;

(d) small-signal

analysis performed

directly on the circuit.

Example

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The hybrid- small-signal model, in its two versions, with

the resistance ro included.

The Early Effect

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Small Signal Equivalent Circuit ModelsDr. MS Ozerdem

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Example - ? -Dr. MS Ozerdem


A common-emitter amplifier

Equivalent circuit obtained by

replacing the transistor with

its hybrid- model.

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(a) A common-emitter amplifier with an emitter resistance Re.

(b) Equivalent circuit obtained by replacing the transistor with its T

model.

A common-emitter amplifier with an emitter resistance Re. Dr. MS Ozerdem


(a) A common-base amplifier

(b) Equivalent circuit obtained by replacing the transistor with its T

model.

A common-base amplifier

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(b) Small-signal equivalent circuit

of the emitter follower with the

transistor replaced by its T model

augmented with ro.

(c) The circuit in (b) redrawn to

emphasize that ro is in parallel

with RL. This simplifies the

analysis considerably.

An emitter-follower circuitDr. MS Ozerdem


(a) An equivalent circuit of the emitter follower obtained from the circuit

in Fig. 5.63(c) by reflecting all resistances in the emitter to the base side.

(b) The circuit in (a) after application of Thévenin theorem to the input

circuit composed of vsig, Rsig, and RB.

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(a) An alternate equivalent circuit of the emitter follower obtained by reflecting

all base-circuit resistances to the emitter side. (b) The circuit in (a) after

application of Thévenin theorem to the input circuit composed of vsig, Rsig / ( 1 1), and RB / ( 1 1).

Dr. MS Ozerdem


Thévenin equivalent circuit of the output of the emitter follower (a). This

circuit can be used to find vo and hence the overall voltage gain vo/vsig for

any desired RL.

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Graphical Analysis

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Graphical construction for the determination

of the dc base current in the circuit

Graphical construction for

determining the dc collector

current IC and the collector-to-

emitter voltage VCE in the circuit

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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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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.

Dr. MS Ozerdem


Two obvious schemes for biasing the BJT: (a) by fixing VBE; (b) by fixing

IB. Both result in wide variations in IC and hence in VCE and therefore are

considered to be “bad.” Neither scheme is recommended.

Biasing the BJT for discrete-circuit design

1-Bias arrangement using a single power supply

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Classical biasing for BJTs using a single power supply:

(a) circuit;

(b) circuit with the voltage divider supplying the base

replaced with its Thévenin equivalent.

Dr. MS Ozerdem


Biasing the BJT using two power supplies.

Resistor RB is needed only if the signal is to be

capacitively coupled to the base. Otherwise, the

base can be connected directly to ground, or to

a grounded signal source, resulting in almost

total -independence of the bias current.


2-Biasing using two power supplies

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(a) A common-emitter transistor amplifier biased by a feedback

resistor RB.

(b) Analysis of the circuit in (a).


3-An alternative biasing arrangement

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(a) A BJT biased using a constant-current source I.

(b) Circuit for implementing the current source I.


4-Biasing using a current source

Dr. MS Ozerdem

Documents

Bipolar Junction Transistors (BJTs)dicle.edu.tr/akademikpersonel/sozerdem/EEZ204/BJTs_ch3.pdf5 Microelectronic Circuits - Fifth Edition Sedra/Smith Copyright 2004 by Oxford University