bipolar transistors - Pearson

heartlustΗλεκτρονική - Συσκευές

2 Νοε 2013 (πριν από 3 χρόνια και 7 μήνες)

71 εμφανίσεις

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.
‹#›

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.
‹#›

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

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›


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


Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›


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

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›


Construction


two polarities:

npn

and
pnp

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›

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

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›


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

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›


Transistor action

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›

Bipolar Transistor Characteristics


Behaviour can be described by the
current gain,
h
fe

or by the
transconductance,
g
m

of the device

21.4

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›


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


Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›


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

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›


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

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›


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

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›


Equivalent circuits for a bipolar transistor


Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›

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

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›

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

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›


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


Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›


Negative feedback amplifiers


Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›

Example



see
Example 21.2

from course text


Determine the

quiescent output

voltage of this

circuit


Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›

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


Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›


A common
-
collector amplifier


unity gain


high input resistance


low output resistance


a very good

buffer amplifier

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›

Other Bipolar Transistor Applications


A phase splitter

21.7

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›


A voltage regulator

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›


A logical switch

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004

OHT 21.
‹#›

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