eithel

B i p o l a r T r a n s i s t o rB a s i u material,
s e m i c o n d u c t o r

pieces of If w e
from two c h a r a c t e r i s t i c s .

diodes are
made up and
that simple their properties in
tutorials w e
saw learnt a b o u t together
Diode and w e also connected
PN-junction PN-junctions
In the simple us two
to form a this will give two junction,
or
germanium
diodes
back-to-back,
a three layer,
silicon
individual signal diodes produces
two
two of t h e s e
now join together N
The fusion
terminal.
short.
common
P or
BJT for
a Transistor, or
that share Bipolar
series
the basis of a
forming either a n
device can act a s
terminal materials that
three semiconductor

different between these

from
terminal active
d e v i c e s made
transistor's ability to change
a r e three voltage. The
Transistors
of a small signal "amplification"
(analogue
conductor by the application electronics)
or

insulator or
a
"switching" (digital
functions:
two basic different regions:
enables it to have within three
two states to operate
transistors have the ability
Then bipolar
electronics).

amplifier and
Ic =
B.Ib
an
transistor operates a s
the
1. Active Region
-

I(saturation)
switch and lc =

as a
the
transistor is "fully-ON" operating
Saturation
2.
switch and lc =
0
as a
operating
the transistor is "fully-OFF"
3. Cut-ofí -

Typical Bipolar Transistor

to describe their
a combination of the words Transfer Varistor used
The word Transistor is an acronym, and is
basic types of bipolar transistor
of development. There are two
mode of operation way back in their early days
construction, NPN and PNP, which basically describes the physical
arrangement of the P-type and N-type

semiconductor materials from which they are made.

The Bipolar Transistor basic construction consists of two PN-junctions producing three connecting terminals with

each teminal being given a name to identity it from the other two. These three terminals are known and labelled as

he Emitler (E ), the Base ( B )and the Collector ( C) respectively.

 Bipolar Transistors are current regulating devices that control the
amount of current
proportion to the amount of biasing voltage applied to their base
flowing through them in
terminal acting like
principle operation of the two transistor
of a
current-controlled switch. The
types NPN and PNP, is
biasing and the polarity of the power supply for exactly the same the only difference being in their
each type.
1. Majority charge carriers- electrons
Bipolar Transistor Construction 2. current flows from collector n type to emitter n type
3. small i is applied to emitter base for large current flow from C to E
4. arrow pointing outwards from the base, indicates flow of electrons
PNP Transistor NPN Transistor
Emitter Callector Emitter Collector
- P N P N P N

Base a). GonStruciion Base

B b). Two-diade Analagy

VE VE
E

V® V® ween
wwwe
C). Symbos

above with the arrow in

the NPN and PNP bipolar transistor are given
for both
The construction and circuit symbols
current flow" between
the base terminal and its
the direction of "conventional
the circuit symbol always showing
to the negative N-type
from the positive P-type region
of the arrow always points
emitter terminal. The direction
for the standard diode symbol.
s a m e as
region for both transistor types, exactly the

Bipolar Transistor Configurations

 As the Bipolar Transistor is a three terminal device,
there are
basically three possible
electronic circuit with one terminal being common to ways to connect it within
both the input and output. Each method
an
responding differently to its input signal within a circuit
of
connection
as the static
circuit arrangement. characteristics of the transistor
vary with each

.1. Common Base

Configuration has Voltage Gain but no Current Gain.
2. Common Emitter Configuration -

has both Current and

Voltage Gain. High Current gain and power gain and
low volage gain as compared to CB
3. Common Collector
Configuration has Current Gain but no
Voltage Gain.

The Common Base (CB) Confiquration

As its name suggests, in the Common Base or
grounded base
configuration, the
BASE connection is common to
both the input signal AND the output signal with the
input signal being applied between the base and the
terminals. The corresponding emitter
output signal is taken from between the base and
the collector terminals as shown
the base terminal with
grounded or connected to a fixed reference
is
voltage point. The input current flowing into the emitter
quite large as its the sum of both the base current and
collector current respectively
output is less than the emitter current input therefore, the collector current
resulting in a current gain for this type of circuit of "1"
other words the common base (unity) or less, in
configuration "attenuates" the input signal.
Ie>> Ib+Ic
The Common Base Transistor Circuit Ic<<<<Ie. current gain ==1

PNP
Rin
Ic

V RL

VEE Va:
This type of amplifier configuration is a non-inverting voltage amplifier circuit, in that the
signal voltages Vin and Vout
are in-phase. This type of transistor arrangement is not verycommo due to its unusually high voltage gain
characteristics. Its output characteristics represent that of a forward biased diode while the input characterisics
represent that of an iluminated photo-diode. Also this type of bipolar transistor configuration has a high ratio of output
to input resistance or more importantly "load" resistance (RL) to "input" resistance (Rin) giving it a valueof

Resistance Gain". Then the voltage gain (Av for a common base configuration is therefore given as:

Common Base Voltage Gain

 A = Vout R
Ay Vin xRIN

The common base circuit is generally only used in single stage amplifier circuits such
radio frequency (R?) amplifiers due to its very
as
microphone pre-amplifier or
good high frequency response.

The Common Emitter (CE) Configuration

In the Common Emitter or
grounded emitter configuration, the input signal is applied between the base, while the
output is taken from between the collector and the
emitter as shown. This type of configuration is the most
commonly
used circuit for transistor based amplifiers and which represents the "normal" method of bipolar transistor connection.
The common emitter amplifier configuration produces the highest current and power gain of all the three
bipolar
transistor configurations. This is mainly because the
input impedance is LOW as it is connected to a forward-biased
PN-junction, while the output impedance is HIGH as it is taken from a reverse-biased PN-junction.

TheCommon Emitter Amplifier Circuit

NPN

RIn

V
R

VsE

In this type of configuration, the current flowing out of the transistor must be equal to the currents flowing into the

transistor as the emitter current is given as le = lc + lb. Also, as the load resistance (RL) is connected in series with

the collector, the current gain of the common emitter transistor configuration is quite large as it is the ratio of lcilb and

Is given the Greek symbol of Beta, (B). As the emitter current for a common emitter configuration is defined as

e = Ic + Ib, the ratio of lc/le is called Alpha, given the Greek symbol of a. Note: that the value of Alpha will always

be less than unity.

ince the electrical relationship between these three currents, lb, Ic and le is determined by the physical construction
will result in a much larger change in the collector
Or the transistor itself, any small change in the base current (lb),
 current (ic). Then, small changes in current flowing in the base will thus control the current in the emitter-collector
circuit. Typically, Beta has a value between 20 and 200 for most general purpose transistors.

Bycombining the expressions for both Alpha, a and Beta, the mathematical relationship between these
parameters and therefore the current gain of the transistor can be given as:

Ie=Ib+Ic, alpha=Ic/Ib+Ic, divide num and den by Ib

alpha=(Ic/Ib)/(1+Ic/Ib)

Alpha,(a)= and Beta(B)=¢

c a.Je = B.I

as: a=
B+1 1-

Where: "Ic" is the current flowing into the collector terminal, "Ib" is the current flowing into the base terminal and "le"
is the current flowing out of the emitter terminal.

Then to summarise, this type of bipolar transistor configuration has a greater input impedance, current and power
gain than that of the common base configuration but its voltage gain is much lower. The common emitter

configuration is an inverting amplifier circuit resulting in the output signal being9 180 out-of-phase with the input
voltage signal.

The Common Collector (CC) Confiquration

In the Common Collector or grounded collector configuration, the collector is now common through the supply. The
input signal is connected directly to the base, while the output is taken from the emitter load as shown. This type of

configuration is commonly known as a Voltage Follower or Emitter Follower circuit. The emitter follower
configuration is very useful for impedance matching applications because of the very high input impedance, in the

region of hundreds of thousands of Ohms while having a relatively low output impedance.

TheCommon Collector Transistor Circuit

 Rin

Is
Ve VcE

Vt

The common emitter configuration has a current gain approximately equal to the ß value of the transistor itself. In the
common collector configuration the load resistance is
situated in series with the emitter so its current is equal to that
of the emitter current. As the emitter current is the
combination of the collector AND the base current combined, the
load resistance in this type of transistor
configuration also has both the collector current and the input current of the
base flowing through it. Then the current gain of the circuit is
givenas:

The Common Collector Current Gain

A-Ec+

A c+1
A, =
B +1

Inis type of bipolar transistor configuration is a non-inverting circuit in that the signal voltages of Vin and Vout are in-

Pnase. It has a voltage gain that is always less than "1" (unity). The load resistance of the common collector transistor

eceives both the base and collector currents giving a large current gain (as with the common emitter configuration)

therefore, providing good current amplification with very litle voltage gain.
 Bipolar Transistor Summary
Then to summarise, the behaviour of the
bipolar transistor in each one of the above circuit
different and
configurations is very
produces different circuit characteristics with regards to input
impedance, output impedance and gain
whether this is voltage gain, current gain or
power gain and this is summarised in the table below.

Bipolar Transistor Characteristics

The static characteristics for a Bipolar Transistor can be divided into the
following three main groups.

Input Characteristics: Common Base AVEBAlE

Common Emitter AVsE/ Al

Output Characteristics: Common Base AVc/Alc

Common Emitter -
AVc Alc
Transfer Characteristics: Common Base Alc/AlE
Common Emitter Alc/Al

with the characteristics of the different transistor configurations given in the following table:

Characteristic Common Common Common

Base Emitter Collector
Input Impedance Low Medium High
Output Impedance Very High High Low
Phase Angle 0 180
Voltage Gain
*****
High **********
Mediumn LOW

niomannunre
Current Gain LoW
***********************
Mediumn *******
High
Power Gain LOW Very High Medium
**********

In the next tutorial about Bipolar Transistors, we willook at the NPN Transistor in more detail when used in the
common emiter configuration as an ampliffer as this is the most widely used configuration due to its flexibility and
high gain. Wa willalso plot the output characteristics curves commonly associated with amplifier circuits as a function

of the collector current to the base current

The NPN Transistor

 that the standard Bipolar Transistor or BJT, comes in two basic forms. An NPN
In the previous tutorial we saw

(Negative-Positive-Negative) type and a PNP (Positive-Negative-Positive) type, with the most commonly used
the transistor junctions can be biased in of three
transistor type being the NPN Transistor. We also learnt that
one

different ways - Common Base, Common Emitter and Common Collector. In this tutorial we willook more closely

configuration using NPN Transistors with example of the construction of a NPN

at the "Common Emitter"
an

transistor along with the transistors current flow characteristics is given below.

An NPN Transistor Confiquration

Circuit
Emitter Collector Symbol
Ic
NP N

Base
Va
V
-

Note: Conventional current flow.

"current" operated device (Beta model) and that a large current ( Ic ) flows freely
We know that the transistor is a

the emitter terminals when the transistor is switched "fully-ON".

through the device between the collector and
transistor at the
current ( Ib ) is flowing into the base terminal of the
However, this only happens when a small biasing
The transistor current in an NPN transistor
as a sort of current control input.
same time thus allowing the Base to act
the symbol of hfe or
the DC Current Gain of the device and is given
is the ratio these two currents (Ic/lb ), called
of
and it is this large ratio between
be large up to 200 for standard transistors,
nowadays Beta. B). The value of ß can
(
device when used in its active region as ib provides the
C and lb that makes the NPN transistor a useful amplifying
a ratio.
that Beta has no units as it is
input and lc provides the output. Note

the Emitter terminal, Ic/le, is called Alpha, ( a ).

Also, the of the transistor from the Collector terminal to
gain
current
across the junction). As the
emitter current le is the
and is a function of the transistor itself (electrons diffusing
collector current, the value of alpha a,
is very close to unity.
product of a very small base current plus a very large
from about 0.950 to 0.999
transistor this value ranges
ana 1or a typical low-power signal

g and Relationship in a NPN Transistor

 tn the orevious tutorial we saw that the standard
Bipolar Transistor or BJT, comes in two basic
lenative-Positive-Negative) type and a PNP
(Positive-Negative-Positive)
type, with the most
forms. An NPN
transistor type being the NPN Transistor. We also learnt that
the transistor
commonly used
different ways Common Base, Common Emitter and
-
junctions can be biased in
one of three
Common Collector. In this
tutorial we will look more
at the "Common Emitter" configuration closely
using NPN Transistors with an
example of the construction of a
NPN
transistor along with the transistors current flow characteristics is
given below.

An NPN Transistor Confiquration

Emitter Circuit
Collector
Symbol
N P N
Base

E
Note: Conventional current flow.

We know that the transistor is a "current" operated device (Beta model) and that a large current ( lc ) flows freely
through the device between the collector and the emitter terminals when the transistor is switched "fully-ON".

However, this only happens when a small biasing current ( tb ) is flowing into the base terminal of the transistor at the
same time thus allowing the Base to act as a sort of current control input. The transistor current in an NPN transistor
is the ratio of these two currents ( Ic/lb ), called the DC Current Gain of the device and is given the symbol of hfe or

nowadays Beta, ( B). The value of Bcan be large up to 200 for standard transistors, and it is this large ratio between
lc and Ib that makes the NPN transistor a useful amplífying device when used in its active region as Ib provides the

input and Ic provides the output. Note that Beta has no units as it is a ratio.

Also, the current gain of the transistor from the Collector terminal to the Emitter terminal, Ic/le, is called Alpha, ( a ),

and is a function of the transistor itself (electrons diffusing across the junction). As the emitter current le is the

product ofa very small base current plus a very large collector current, the value of alpha a, is very close to unity,

and for a typical low-power signal transistor this value ranges from about 0.950 to 0.999

gand BRelationship in a NPN Transistor

 DC Current Gain = Output Current
Input Current
I l +
lc (KCL) and
=
Thus: Ig =Ig + Ic Ic
and Igle 1-

. 1

1-
By combining the two parameters and
a B we can produce two mathematical expressions that
between the different currents gives the relationship
flowing in the transistor.

B= 1-O and
B+T
0.99
If = =0.99 B = 99
99

ne values of Beta
vary from about 20 for high current power transistors to well over 1000 for
power type
high frequency low
bipolar transistors. The value of Beta for most standard NPN transistors can be
found in the
manufactures datasheets but generally range between 50-200.

Equation above for Beta can also be re-arranged to make lc as the subject, and with a zero base current ( Ib
he
resultant collector current Ic will also be zero. ( Bx0 ). Also when the base current is high the corresponding
 collector current will also be high resulting in the base current
controlling the collector current. One of the most
important properties of the Bipolar Junction Transistor is that a small base
current can control a much
collector current. Consider the larger
following example.

Example No1
An NPN Transistor has a DC current gain, (Beta) value of 200. Calculate the base current lb
resistive load of 4mA. required to switch a

B= 4x10
200
=
20uA
Therefore, B =200, lc =
4mA and lb =20pA
One other point to remember
about NPN Transistors. The
collector voltage. (Vc ) must be
respect to the emitter voltage, ( Ve ) to allow greater and positive with
current to flow
through the transistor between the collector-emitter
junctions. Also, there is a voltage drop between the
Base and the Emitter
for silicon devices as the terminal of about 0.7v (one diode volt
input characteristics of an NPN Transistor drop)
are of a forward
voltage. ( Vbe ) of a NPN transistor must be biased diode. Then the base
greater than this 0.7V otherwise the transistor
base current will not conduct with the
given as.

B-BE
Rp
Where: Ib is the base
current, Vb is the base bias
base input resistor.
voltage, Vbe is the base-emitter
volt drop
Increasing lb, Vbe (0.7v) and Rb is the
slowly increases to 0.7V but Ic rises
exponentially.
Example No2
An NPN Transistor has a DC base bias
voltage, Vb of 10v and an input base resistor, Rb
value of the base of 100kQ. What will
current into the transistor. be the

In=BBE =10-0.7g3
100k 93uA
Therefore, lb 93uA.