Bipolar Junction Transistor (BJT)

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CHAPTER 4


Bipolar Junction Transistor

(BJT)







OBJECTIVES


Describe the basic structure of the bipolar junction
transistor (BJT).



Explain and analyze basic transistor bias and operation.



Discuss how a transistor can be used as an amplifier or
a switch.



Discuss the parameters and characteristics of a
transistor and how they apply to transistor circuits.

Basic Structure


A transistor is a device that can be used
as either an amplifier or a switch.


Has 3 terminals:


Base, B


Collector, C


Emitter, E


Bipolar since conduction current is due to
both the majority and minority carriers.

BJT

4.1

Basic Operation

4.2

Configuration and current relationship

4.3

Operational regions

4.4

DC load line

4.5

BJT Device specification & Data Sheet

4.6

Design & analysis of biasing circuits

4.7

Bias Stability

4.8

Amplification Concept (graphical)


2 types:
-

npn

and
pnp


There are three
layers and two p
-
n
junctions.


Transistors can be
either
pnp

or
npn

type.


Basic Structure


Arrow points from p to n.


Base is very thin with very small
amount of majority carrier.


Emitter is wider with the most
number of majority carrier.


Collector is wider than emitter
but with little majority carrier.


Emitter : emits charge


Collector : collects charge


Base : a gate that controls
flow of current

Basic Structure

-

npn


Arrow marks emitter points
from p to n


Base layer is thin with little
majority carrier.


Emitter is thicker with the
most majority carrier


Collector is thickest with little
majority carrier.

Basic Structure

-

pnp

Diode Analogy


npn

transistor





pnp

transistor

Cross Section of Integrated

Circuit
npn

Transistor



Operation

4 Possible Modes of Operation

1.
Forward
-
Active
-

amplifier



B
-
E junction is forward biased



B
-
C junction is reverse biased

2.

Saturation


switch (on)



B
-
E and B
-
C junctions are forward biased

3.

Cut
-
Off


switch (off)



B
-
E and B
-
C junctions are reverse biased

4.

Inverse
-
Active (or Reverse
-
Active)


(off)



B
-
E junction is reverse biased



B
-
C junction is forward biased

Forward
-
Active (
npn
)


Transistor currents in BJT in Forward
-
Active (
npn
)

Cont..


B
-
E junction forward biased


Electrons (majority) from emitter injected into base
through BE junction.


Become minority in base



C
-
B junction reversed biased


Base very thin, therefore get swept across C
-
B
junction to become collector current


Very little electrons left in base to become base
current

BJT

4.1

Basic Operation

4.2

Configuration and current relationship

4.3

Operational regions

4.4

DC load line

4.5

BJT Device specification & Data Sheet

4.6

Design & analysis of biasing circuits

4.7

Bias Stability

4.8

Amplification Concept (graphical)

Current flow in
npn

BJT


Currents in a Transistor


Emitter current is the sum of the
collector and base currents:





The collector current is comprised
of two currents:

Transistor Characteristics

and Parameters


V
BB

forward biases the BE
junction.


V
CC

reverse
-
biases the BC
junction.


The base
-
emitter current
changes yield large changes in
collector
-
emitter current.


The factor of this change is
called beta(
β)
or DC current
gain. (Typical value: 20
-
200)

Cont..


There are 3 key dc voltages and
dc currents to be considered.


I
B
: dc base current


I
E
: dc emitter current


I
C
: dc collector current


V
BE
: dc voltage across base
-
emitter junction


V
CB
: dc voltage across
collector
-
base junction


V
CE
: dc voltage from collector
to emitter

Cont..


For
proper operation
,
base
-
emitter junction is forward
biased by V
BB
.


The
collector
-
base junction
is reverse biased by V
CC

and
blocks current flow
through it’s junction

just like
a diode.



Current flow through the base
-
emitter junction will help
establish the path for current
flow from the collector to emitter.


Application of these laws
begins with the base circuit to
determine the amount of base
current. Using Kirchhoff’s
voltage law,

Cont..



, will be used in
most analysis examples

V
RB

= V
BB



V
BE

Ohm’s Law,

V
RB

= I
B
R
B

So,

I
B

= (V
BB


V
BE
)/R
B

I
C

=
β
I
B



In the collector circuit
-

determine that V
CC

is
distributed proportionally
across R
C

and the
transistor(V
CE
).


Find V
CE

and V
CB
.

Cont..

Problem


Determine I
B
,I
C
,I
E
,V
CE

and V
CB

for the following values:



R
B

= 22 k


R
C

= 220


V
BB

= 6V


V
CC

= 9V


β = 90

Collector Characteristic Curve


Collector characteristic
curves give a graphical
illustration of the relationship
of collector current and V
CE

with specified amounts of I
B
.


With greater increases of
V
CC
, V
CE

continues to
increase until it reaches
breakdown, but the current
remains about the same in
the linear region from 0.7V to
the breakdown voltage.

Cont..


With no I
B

the transistor is in the
cut
-
off region
-

no
current flow in the collector part of the circuit
.


With the transistor in a cut
-
off state the full V
CC

can be
measured across the collector and emitter (V
CE
).


I
CEO

is due to the minority carriers.

Cont..


Once
I
C

maximum is reached
, the
transistor is said to be
in saturation
. Note that saturation can be determined by
application of Ohm’s law. I
C
(sat)=V
CC
/R
C


The measured voltage across, now “shorted” collector
and emitter is 0V.

BJT

4.1

Basic Operation

4.2

Configuration and current relationship

4.3

Operational regions

4.4

DC load line

4.5

BJT Device specification & Data Sheet

4.6

Design & analysis of biasing circuits

4.7

Bias Stability

4.8

Amplification Concept (graphical)

Transistor Configurations

1.
Common Base


The
base is the common terminal
between input and
output

2.
Common Emitter


The
emitter is the common terminal
between input
and output

3.
Common Collector


The
collector is the common terminal
between input
and output

Common
-
Base Configuration


The base is common to both input (emitter

base) and
output (collector

base) of the transistor.

Input Characteristics



This curve shows the
relationship between of
input current (I
E
) to input
voltage (V
BE
) for various
levels of output voltage
(V
CB
).

Common
-
Base Configuration

Output Characteristics



This graph demonstrates
the relationship between
the output current (I
C
) to
an output voltage (V
CB
) for
various levels of input
current (I
E
).

Common
-
Base Configuration


Active region:


BE forward biased, CB reverse biased


I
C

~ I
E

and independent of V
CB


Cut
-
off
:


under I
E

=0. Both BE and CB reverse biased. Only
I
CBO

exist.


Saturation:


Both BE and CB forward biased. Left of V
CB
=0.


Small

changes in V
CB

cause big changes on I
C
.

Common
-
Base Configuration

Operational regions


Active


Operating range of the
amplifier


Cutoff


Amplifier is basically off


There is voltage but little
current.


Saturation


Amplifier is fully on


There is current but little
voltage

Approximation


Emitter and collector
currents:




Base
-
emitter voltage:

Common
-
Base Configuration

Alpha (
α
)


Alpha (
α
) relates the DC currents I
C

and I
E

:




Ideally:
α

= 1


In reality: a is between 0.9 and 0.998



Alpha (
α
) in the AC mode:

Common
-
Base Configuration

Common

Emitter Configuration


The emitter is common to
both input (base
-
emitter) and
output (collector
-
emitter).


The input is the base terminal
and the output is the collector
terminal.


Usually used as an amplifier
circuit.

Input Characteristics




At a fixed V
BE
, I
B

decreases as V
CE

increases. Why?

Common

Emitter Configuration

Output Characteristics



For V
CE

<
V
CEsat
, I
C

increases
linearly.


Once V
CE

>
V
CEsat
, I
C

not
influenced by V
CE
.


I
B

(
μ
A) is small compared to I
C

(
mA
).


Small increase in I
B

causing a big
change in I
C
.


When I
B
=0, I
C

exists which is I
CEO
.

Common

Emitter Configuration

Common
-
Emitter Amplifier Currents


Ideal Currents




Actual Currents




When I
B

= 0
μA

the transistor is in cut
-
off, but there is some
minority current flowing called I
CEO
.

Common

Emitter Configuration

where I
CBO

= minority collector current.

This is usually so small that it can be
ignored, except in high power transistors
and in high temperature environments.

Beta (
β
)


β

represents the amplification factor of a transistor. (
β

is
sometimes referred to as
h
fe
, a term used in transistor
modeling calculations)





In DC mode:





In AC mode:

Common

Emitter Configuration

(V
CE

= constant)


Common

Emitter Configuration

Beta (
β)


Relationship between amplification factors
β

and
α
:




Relationship Between Currents:

Common

Emitter Configuration








Input is the base terminal and output is the
emitter terminal

Common

collector Configuration


The characteristics are similar to those of the common
-
emitter configuration, except the vertical axis is I
E
.

Common

collector Configuration

Limitations of Operation for Each Configuration

Common

collector Configuration



V
CE

is at maximum and I
C

is
at minimum (
I
cmax

= I
CEO
) in the
cutoff region.




I
C

is at maximum and V
CE

is
at minimum (
V
CEmax

=
V
CEsat

=
V
CEO
) in the saturation region.




The transistor operates in the
active region between
saturation and cutoff.

Transistor Switch


When a transistor is used as a switch, it simply being
biased so that it is in
cut
-
off (switched off)
or
saturation
(switched on)
.

Power Dissipation

Common
-
base

Common
-
emitter

Common
-
collector

Power
Derating

Factor

Example:

An
npn

transistor in a circuit has V
CE

=20V. This circuit to be

operated at 125
°
C. Given that the maximum power

dissipation at 25
°
C is 310
mW
.

The
derating

factor is 2.81
mW
/
°
C for temperatures above

25
°
C.Find the new
I
c,max
.

Answer:

Δ
T = 125
°
C


25
°
C = 100
°
C

Derated

Amount = 2.81
mW
/
°
C x 100
°
C = 281mW

Maximum Power Dissipated @ 125
°
C = 310


281 = 29
mW

So, I
C,max

= P
C,max
/V
CE

= 29 mW/20V = 1.45 mA