# bipolar transistors - Pearson

Electronics - Devices

Nov 2, 2013 (4 years and 6 months ago)

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Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›

Bipolar Transistors

Introduction

An Overview of Bipolar Transistors

Bipolar Transistor Operation

Bipolar Transistor Characteristics

Summary of Bipolar Transistor Characteristics

Bipolar Transistor Amplifiers

Other Bipolar Transistor Applications

Chapter 21

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
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Introduction

Bipolar transistors are one of the main

‘building
-
blocks’ in electronic systems

They are used in both analogue and digital circuits

They incorporate two
pn

junctions and are
sometimes known as
bipolar junction transistors

or
BJTs

Here will refer to them simply as
bipolar transistors

21.1

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
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An Overview of Bipolar Transistors

While control in a FET is due to an electric
field
,
control in a bipolar transistor is generally considered
to be due to an electric
current

current into one terminal

determines the current

between two others

as with a FET, a

bipolar transistor

can be used as a

‘control device’

21.2

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OHT 21.
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Notation

Notation

bipolar transistors are 3
terminal devices

collector (c)

base (b)

emitter (e)

the base is the control input

diagram illustrates the
notation used for labelling
voltages and currents

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OHT 21.
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Relationship between the collector current and the
base current in a bipolar transistor

characteristic is

approximately linear

magnitude of collector

current is generally

many times that of the

base current

the device provides

current gain

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OHT 21.
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Construction

two polarities:

npn

and
pnp

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Bipolar Transistor Operation

We will consider
npn

transistors

pnp

devices are similar but with different polarities of
voltage and currents

when using
npn
transistors

collector is normally more positive than the emitter

V
CE

might be a few volts

device resembles two back
-
to
-
back diodes

but has very
different characteristics

with the base open
-
circuit negligible current flows from the
collector to the emitter

21.3

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Now consider what happens when a positive voltage

is applied to the base (with respect to the emitter)

this forward biases the base
-
emitter junction

the base region is light doped and very thin

because it is likely doped, the current produced is

mainly electrons flowing from the emitter to the base

because the base region is thin, most of the electrons
entering the base get swept across the base
-
collector
junction into the collector

this produces a collector current that is much larger than
the base current

this gives
current amplification

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Transistor action

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OHT 21.
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Bipolar Transistor Characteristics

Behaviour can be described by the
current gain,
h
fe

or by the
transconductance,
g
m

of the device

21.4

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Transistor configurations

transistors can be used in a
number of configurations

most common is as shown

emitter

terminal is common
to input and output circuits

this is a
common
-
emitter

configuration

we will look at the
characteristics of the device
in this configuration

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Input characteristics

the input takes the
form of a forward
-
biased
pn

junction

the input
characteristics are
therefore similar to
those of a
semiconductor diode

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OHT 21.
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Output characteristics

region near to the

origin is the

saturation region

this is normally

avoided in linear

circuits

slope of lines

represents the

output resistance

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Transfer characteristics

can be described by either the current gain or by the
transconductance

DC current gain
h
FE

or

is given by

I
C
/ I
B

AC current gain

h
fe

is given by

i
c
/ i
b

transconductance
g
m

is given approximately by

g
m

40
I
C

40
I
E

siemens

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Equivalent circuits for a bipolar transistor

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Summary of Bipolar Transistor Characteristics

Bipolar transistors have three terminals: collector,
base and emitter

The base is the control input

Two polarities of device:
npn

and
pnp

The collector current is controlled by the base
voltage/current
I
C

=
h
FE
I
B

Behaviour is characterised by the current gain or the
transconductance

21.5

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Bipolar Transistor Amplifiers

A simple transistor amplifier

R
B

is used to ‘
bias
’ the

transistor by injecting an

appropriate base current

C

is a
coupling capacitor

and is used to couple the

AC signal while preventing

external circuits from

affecting the bias

this is an
AC
-
coupled amplifier

21.6

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AC
-
coupled amplifier

V
B

is set by the conduction voltage of the base
-
emitter
junction and so is about 0.7 V

voltage across
R
B

is thus
V
CC

0.7

this voltage divided by
R
B

gives the base current
I
B

the collector current is then given by
I
C

=
h
FE
I
B

the voltage drop across
R
C

is given by
I
C
R
C

the
quiescent output voltage

is therefore

V
o

=
V
CC

-

I
C
R
C

output is determined by
h
FE
which is very variable

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OHT 21.
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Negative feedback amplifiers

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Example

see
Example 21.2

from course text

Determine the

quiescent output

voltage of this

circuit

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Base current

is small, so

Emitter voltage

V
E

=
V
B

V
BE

= 2.7

0.7 = 2.0 V

Emitter current

Since
I
B

is small,
collector current

I
C

I
E
= 2 mA

Output voltage

=
V
CC

I
C
R
C

=
10
-

2 mA

2.2 k

= 5.6 V

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OHT 21.
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A common
-
collector amplifier

unity gain

high input resistance

low output resistance

a very good

buffer amplifier

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Other Bipolar Transistor Applications

A phase splitter

21.7

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A voltage regulator

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A logical switch

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Key Points

Bipolar transistors are widely used in both analogue and
digital circuits

They can be considered as either voltage
-
controlled or
current
-
controlled devices

Their characteristics may be described by their gain or by
their transconductance

Feedback can be used to overcome problems of variability

The majority of circuits use transistors in a common
-
emitter
configuration where the input is applied to the base and the
output is taken from the collector

Common
-
collector circuits make good buffer amplifiers

Bipolar transistors are used in a wide range of applications