chapter 3 & 4: bipolar junction transistors and...

Chapter 3 & 4:

Bipolar Junction Transistors and Applications© Modified by Yuttapong Jiraraksopakun

ENE, KMUTT 2009

Copyright ©2009 by Pearson Education, Inc.

Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/e

Robert L. Boylestad and Louis Nashelsky

Transistor Construction

There are two types of transistors:

• pnp

• npn

The terminals are labeled:

• E - Emitter

• B - Base

• C - Collector

pnp

npn

2





Transistor OperationWith the external sources, V

EEand V

CC, connected as shown:

• The emitter-base junction is forward biased

• The base-collector junction is reverse biased

3





Currents in a Transistor

The collector current is comprised of two

currents:

BI

CI

EI +=

minorityCOI

majorityCI

CI +=

Emitter current is the sum of the collector and

base currents:

4





Common-Base Configuration

The base is common to both input (emitter–base) and

output (collector–base) of the transistor.

5





Common-Base Amplifier

Input Characteristics

This curve shows the relationship

between of input current (IE) to input

voltage (VBE) for three output voltage

(VCB) levels.

6





This graph demonstrates

the output current (IC) to

an output voltage (VCB) for

various levels of input

current (IE).

Common-Base Amplifier

Output Characteristics

7

CBOI

COI =





Operating Regions

• Active – Operating range of the

amplifier.

• Cutoff – The amplifier is basically

off. There is voltage, but little

current.

• Saturation – The amplifier is full on.

There is current, but little voltage.

8

Regions Base-Emitter Collector-Base

Active Forward-biased Reverse-biased

Cutoff Reverse-biased Reverse-biased

Saturation Forward-biased Forward-biased





EI

CI ≅

Silicon)(for V 0.7BEV =

Approximations

Emitter and collector currents:

Base-emitter voltage:

9





Ideally: α = 1

In reality: α is between 0.9 and 0.998

Alpha (α)

Alpha (α) is the ratio of ICto I

E:

EI

CI

α =

dc

Alpha (α) in the AC mode:

EI

CI

α

∆

∆

ac =

10

constant=

CBV

CBOI

EαI

CI +=





Transistor Amplification

Voltage Gain:

V 50kΩ 5ma 10

mA 10

10mA

20Ω

200mV

===

=≅

≅

====

))((RL

IL

V

iI

LI

EI

CI

iR

iV

iI

EI

Currents and Voltages:

11

250

200mV

50V===

iV

LV

vA

Omit DC biasing to demonstrate AC response

Assume Riand R

ofrom input & output

characteristic curves





Common–Emitter Configuration

The emitter is common to both input

(base-emitter) and output (collector-

emitter).

The input is on the base and the

output is on the collector.

12





Common-Emitter Characteristics

Collector Characteristics Base Characteristics

13





Common-Emitter Amplifier Currents

Ideal Currents

IE= I

C+ I

BIC= α I

E

Actual Currents

IC= α I

E+ I

CBO

When IB= 0 µA the transistor is in cutoff, but there is some minority

current flowing called ICEO

.

µA 0=−

=BI

CBO

CEO

α

II

1

where ICBO

= minority collector current

14

ICBO is usually so small that it can be ignored, except in high

power transistors and in high temperature environments.





Beta (β)

In DC mode:

In AC mode:

β represents the amplification factor of a transistor. (β is

sometimes referred to as hfe, a term used in transistor modeling

calculations)

B

C

I

Iβ =

dc

constantac =

∆

∆=

CEV

B

C

I

Iβ

15





Determining β from a Graph

Beta (β)

108

A 25

mA 2.7β 7.5VDC CE

=

µ=

=

100

µA 10

mA 1

µA) 20 µA (30

mA) 2.2mA (3.2β

7.5V

AC

CE

=

=

−

−

=

=

16

Both β values are usually reasonably close and are often used interchangeably





Relationship between amplification factors β and α

1β

βα

+

=

1α

αβ

−

=

Beta (β)

Relationship Between Currents

BC βII =BE 1)I(βI +=

17





Common–Collector Configuration

The input is on the

base and the output is

on the emitter.

18





Common–Collector Configuration

The characteristics are

similar to those of the

common-emitter

configuration, except the

vertical axis is IE.

19





VCEis at maximum and I

Cis at

minimum (ICmax

= ICEO

) in the cutoff

region.

ICis at maximum and V

CEis at

minimum (VCE max

= VCEsat

= VCEO

) in

the saturation region.

The transistor operates in the active

region between saturation and cutoff.

Operating Limits for Each Configuration

20





Power Dissipation

Common-collector:

CCBCmax IVP =

CCECmax IVP =

ECECmax IVP =

Common-base:

Common-emitter:

21





Transistor Specification Sheet

22





Transistor Specification Sheet

23





Transistor Terminal Identification

24





Biasing

Biasing: The DC voltages applied to a transistor in

order to turn it on so that it can amplify the AC signal.

( )

BC

CBE

BE

II

II1I

V7.0V

β

β

=

≅+=

=





Operating Point

The DC input

establishes an

operating or

quiescent point

called the Q-point.





The Three States of Operation

• Active or Linear Region Operation

Base–Emitter junction is forward biased

Base–Collector junction is reverse biased

• Cutoff Region Operation

Base–Emitter junction is reverse biased

• Saturation Region Operation

Base–Emitter junction is forward biased

Base–Collector junction is forward biased





DC Biasing Circuits

• Fixed-bias circuit

• Emitter-stabilized bias circuit

• Collector-emitter loop

• Voltage divider bias circuit

• DC bias with voltage feedback





Fixed Bias





The Base-Emitter Loop

From Kirchhoff’s voltage

law:

Solving for base current:

+VCC– I

BR

B– V

BE= 0

B

BECCB

R

VVI

−

=





Collector-Emitter Loop

Collector current:

From Kirchhoff’s voltage law:

BIICβ=

CCCCCE RIVV −=





Saturation

When the transistor is operating in saturation, current

through the transistor is at its maximum possible value.

CR

CCV

CsatI =

V 0CEV ≅

This approximation is equivalent to move the region below

VCEsat

of the output curves to align on the output current

axis.





Load Line Analysis

ICsat

IC= V

CC/ R

C

VCE= 0 V

VCEcutoff

VCE= V

CC

IC= 0 mA

• where the value of RBsets the value of

IB

• that sets the values of VCE

and IC

The Q-point is the operating point:

The end points of the load line are:





Circuit Values Affect the Q-Point

more …





Circuit Values Affect the Q-Point





Emitter-Stabilized Bias Circuit

Adding a resistor

(RE) to the emitter

circuit stabilizes

the bias circuit.





Base-Emitter Loop


This image cannot currently be displayed.

0R1)I(β-V-RI-VEBBEBBCC=+

0 RI-V-RI-VEEBEBBCC=+

EB

BECCB

1)R(R

V-VI

+β+=

Since IE= (β + 1)I

B:

Solving for IB:







0 CC

VC

RC

I CE

V E

RE

I =−++

Since IE≅ I

C:

)R (RI– V V ECCCCCE +=

Also:

This image cannot currently be displayed.

EBEBRCCB

CCCCECEC

EEE

V V RI– V V

RI - V V V V

RI V

+==

=+=

=





Improved Biased Stability

Stability refers to a circuit condition in which the currents and voltages

will remain fairly constant over a wide range of temperatures and

transistor Beta (β) values.

Adding RE to the emitter improves the stability of a transistor.

EB

BECCB

1)R(R

V-VI

+β+=

B

BECCB

R

VVI

−

=

BIICβ=

Fixed-bias circuit Emitter-stabilized bias circuit

IBin fixed-bias circuit cannot change, so change in β results in large

change in output current and voltage.





Saturation Level

VCEcutoff

: ICsat:

The endpoints can be determined from the load line.

mA 0 I

V V

C

CCCE

=

=

ERCR

CCV

CI

CE V 0V

+

=

=





Voltage Divider Bias

This is a very stable

bias circuit.

The currents and

voltages are nearly

independent of any

variations in β.





Approximate Analysis

Where IB<< I

1and I

1≅ I

2 :

Where βRE> 10R

2:


21

CC2B

RR

VRV

+

=

E

EE

R

VI =

BEBE VVV −=

EECCCCCERI RI V V −−=

)R (RIV V

II

ECCCCCE

CE

+−=

≅





Voltage Divider Bias Analysis

Transistor Saturation Level

EC

CCCmaxCsat

RR

VII

+

==

Load Line Analysis

Cutoff: Saturation:

mA0I

VV

C

CCCE

=

=

V0VCE

ER

CR

CCV

CI

=

+

=





Voltage Divider Bias





Voltage Divider Bias (Exact)





DC Bias with Voltage Feedback

Another way to

improve the stability

of a bias circuit is to

add a feedback path

from collector to

base.

In this bias circuit

the Q-point is only

slightly dependent on

the transistor beta, β.





Base-Emitter Loop

)R(RR

VVI

ECB

BECCB

+β+

−=


0RI–V–RI–RI– V EEBEBBCCCC =′

Where IB<< I

C:

CI

BI

CI

CI' ≅+=

Knowing IC= βI

Band I

E≅ I

C, the loop

equation becomes:

0RIVRIRI– V EBBEBBCBCC =β−−−β

Solving for IB:






Applying Kirchoff’s voltage law:

IE+ V

CE+ I’

CR

C– V

CC= 0

Since I′C≅ I

Cand I

C= βI

B:

IC(R

C+ R

E)+ V

CE– V

CC=0

Solving for VCE:

VCE= V

CC– I

C(R

C+ R

E)





Base-Emitter Bias Analysis

Transistor Saturation Level

EC

CCCmaxCsat

RR

VII

+

==

Load Line Analysis

Cutoff: Saturation:

mA 0I

VV

C

CCCE

=

=

V 0VCE

ER

CR

CCV

CI

=

+

=





Transistor Switching Networks

Transistors with only the DC source applied can be used

as electronic switches.





Switching Circuit Calculations

C

CCCsat

R

VI =

dc

CsatB

II

β>

Csat

CEsatsat

I

VR =

CEO

CCcutoff

I

VR =

Saturation current:

To ensure saturation:

Emitter-collector resistance

at saturation and cutoff:





Switching Time

Transistor switching times:

dron ttt +=

fsoff ttt +=





Troubleshooting Hints

• Approximate voltages

– VBE

≅ .7 V for silicon transistors

– VCE

≅ 25% to 75% of VCC

• Test for opens and shorts with an ohmmeter.

• Test the solder joints.

• Test the transistor with a transistor tester or a curve tracer.

• Note that the load or the next stage affects the transistor operation.





PNP Transistors

The analysis for pnp transistor biasing circuits is the same

as that for npn transistor circuits. The only difference is that

the currents are flowing in the opposite direction.





Homework 3 (Chapter 3)

• Common-Base Configuration

– 3.4 (13)

• Transistor Amplifying Action

– 3.5 (18)

• Common-Emitter Configuration

– 3.6 (23)

56





Homework 3 (Chapter 4)

• Fixed-Bias Configuration

– 4.3 (1)

• Emitter-Bias Configuration

– 4.4 (8)

• Voltage Divider Configuration

– 4.5 (13)

• Collector-Feedback Configuration

– 4.6 (23)

• Miscellaneous Bias Configuration

– 4.59 (30)

57

chapter 3 & 4: bipolar junction transistors and...

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