EGR 240 Mechatronics and Smart System Design Optional Laboratory 08: Working with Transistor Switches Goal of this lab: To gain hands on experience using transistors as electrically controlled switches. Section A: Read the following background information on transistors.

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E
GR
240
Mechatronic
s and Smart System Design

Optional
Lab
oratory 0
8
:
Working with Transistor Switches


Goal of this lab: To gain hands on experience using transistors as electrically
controlled switches.



Section A: Read the following background inform
ation on transistors.


Transistor Background


Working with Transistors


Section B:
Breadboarding Transistor Circuits


S
tep 1: Build an input switch and Digital Inverter circuit


Step 2: Build an NPN transistor circuit


Step 3
: Build a PNP transistor circuit


Step 4: Determining transistor characteristics


Step 5: Running a motor with a transistor
.


Step 6: Setting up a MOSFET transistor circuit:



Equipment needed:


5 volt power supply


9 volt

battery and connectors


2N3904 BJT transistor


2N3906 BJT transistor


2709A transistor


TIP 120 Transistor


TIP 125 Transistor


Assorted resistors, diodes, and LEDs


12 volt lamp bulb


740
4

inverter gate

IC
.


IRLZ
4
4N and 2N7000 MOSFET transistors


Breadboard


small and medium 9 Volt Motors














E
GR
240 Mechatronics and Smart System Design

Lab 0
8
:
Section A: Transistor Background:


In
S
ection A, you are to simply read the i
nformation
which is intended to give you an
introduction to transistors. The information comes from two different sources:


Transistor Background
: condensed from
http://en.wikipedia.org/wiki/Transistors

and


The Theory of
Working with Transistors
:
modified from
the Electronics Club



webpage at


www.kpsec.freeuk.com
.


Read both of these background features to gain some working background on transistors
as electronically controlled switches.

-
---------------------------------------------------------------------------------------------------------
.

Transistor Background: from
Wikipedia

Transistor

In electronics, a transistor is a semiconductor device commonly
used to amplify or switch electroni
c signals. A transistor is made
of a solid piece of a semiconductor material, with at least three
terminals for connection to an external circuit. A voltage or
current applied to one pair of the transistor's terminals changes
the current flowing through an
other pair of terminals. Because
the controlled (output) power can be much larger than the
controlling (input) power, the transistor provides amplification
of a signal. The transistor is the fundamental building block of
modern electronic devices, and is u
sed in radio, telephone,
computer and other electronic systems. Some transistors are
packaged individually but most are found in integrated circuits.


Importance

The transistor is considered by many to be
the

greatest invention of the twentieth
-
century,
[2]

or as one of the greatest.
[3]

It is the key active component in practical
ly all
modern
electronics
. Its importance in today's society rests on its ability to be
mass
produced

using a highly automated process (
fabrication
) that achieves astonishingly low
per
-
transistor costs.

Although several companies each produce over a billio
n individually
-
packaged (known
as
discrete
) transistors every year,
[4]

the vast majority of transistors produced are in
integrated circuits

(often shortened to
IC
,
m
icrochips

or simply
chips
) along with
diodes
,
resistors
,
capacitors

and other
electronic components

to produce complete electronic
circuits. A
logic gate

consists of about twenty transistors whereas an advanced
microprocessor, as of 2006, can use as many a
s 1.7 billion transistors (
MOSFETs
).
[5]




Transistor used as

switch in a
grounded
-
emitter circuit.


Standard common
-
emitter

transistor configuration

"About 60 million transistors were built this year [2002] ...

for [each] man, woman, and
child on Earth."
[6]

The transistor's low cost, flexibility and reliability have made it a ubiquitous device.
Transistorized
mechatronic

cir
cuits have replaced
electromechanical devices

in
controlling appliances and machinery. It is often easier and cheaper to use a standard
microcontroller

and write a
computer progr
am

to carry out a control function than to
design an equivalent mechanical control function.

Usage

In the early days of transistor circuit design, the
bipolar junction transistor
, or BJT, was
the most commonly used transistor. Even after
MOSFETs

became available, the BJT
remained the transistor of choice for digital and analog circuits because of their ease of
manufacture and speed. However, desirable properties of MOSFETs, such as their utilit
y
in low
-
power devices, have made them the ubiquitous choice for use in digital circuits
and a very common choice for use in analog circuits.


How a transistor works

T
he essential usefulness of a transistor comes from

its
ability to use a small signal app
lied between one pair of its
terminals to control a much larger signal at another pair of
terminals. This property is called "gain". A transistor can
control its output in proportion to the input signal; this is
called an "amplifier". Or, the transistor ca
n be used to
turn current on or off in a circuit like an electrically
controlled "switch", where the amount of current is
determined by other circuit elements.

The two types of transistors have slight differences in
how they are used in a circuit. A bipol
ar transistor

(BJT)

has terminals labeled
base, collector
,

and
emitter.

A
small current at
the b
ase terminal can control or switch a
much larger current between collector and emitter
terminals. For a field
-
effect transistor

(FET)
, the
terminals are labeled

gate, source
, and
drain
, and a
voltage at the gate can control a current between source
and drain.

The image to the right represents a typical bipolar
transistor in a circuit. Charge will flow between emitter
and collector terminals depending on the curre
nt in the


Amplifier circuit using a transistor.


Operation graph of a transistor.


base. Since internally the base and emitter connections behave like a semiconductor
diode, a voltage drop develops between base and emitter while the base current exists.
The size of this voltage depends on the material the transistor is made fro
m, and is
referred to as
V
be
.

Transistor as a switch

Transistors are commonly used as electronic switches,
for both high power applications including
switched
-
mode power supplies

and low power applications such as
logic gates
.

It can be seen from the grap
h that once the base voltage
reaches a certain level, shown at B, no more current will
exist and the output will be held at a fixed voltage. The
transistor is then said to be
saturated
. Hence, values of
input voltage can be chosen such that the output is e
ither
completely off,
[7]

or completely on. The transistor is
acting as a switch, and this type of operation is common
in
digital circuits

where only "on" and "off" v
alues are
r
elevant.

Transistor as an amplifier

The common emitter amplifier is designed so that a small change in voltage in (
V
in
)
changes the small current through the base of the transistor and the transistor's current
amplification combined with the pr
operties of the circuit mean that small swings in
V
in

produce large changes in
V
out
.

It is important that the operating parameters of the
transistor are chosen and the circuit designed such that
as far as possible the transistor operates within a
linear

po
rtion of the graph, such as that shown between A and
B, otherwise the output signal will suffer
distortion
.

Various configurations of single transistor amplifier
are possible, with some providing current gain, some
voltage gain, and some both.

From
mobile
phones

to
televisions
, vast numbers of products include
amplifiers

for
sound
reproduction
,
radio transmission
, and
signal processing
. The first discrete transistor audio
amplifiers barely supplied a few hundred milliwatts, but power and audio fidelity
grad
ually increased as better transistors became available and amplifier architecture
evolved.

Modern transistor audio amplifiers of up to a few hundred
watts

are common and
relatively inexpensive. Transistors have replaced
valves (electron tubes)

in instrumen
t
amplifiers.


Types

of Transistor:

Symbols for different transistors
:


Bipolar Transistor: Field Effect Transistors:




Transistors are categorized by:



Semiconductor material

: germanium, silicon, gallium arsenide, silicon carbide,
etc.



Struct
ure:
BJT, JFET, IGFET (MOSFET), IGBT, "other types
"



Polarity:
NPN, PNP

(BJTs); N
-
channel, P
-
channel (FETs)



Maximum power rating: low, medium, high



Maximum operating frequency: low, medium, high,
radio frequency

(RF),
microwave

(The maximum effective fre
quency of a transistor is denoted by the
term
f
T
, an abbreviation for "frequency of transition". The frequency of transition
is the frequency at which the transistor yields unity gain).



Application: switch, general purpose, audio, high voltage, super
-
beta
, matched
pair



Physical packaging:
through hole

metal, through hole plastic,
surface mount
, ball
grid array, power modules



Amplification factor
h
fe

(transistor beta)
[8]


Thus, a particular transistor may be described as:
silicon, surface mount, BJT, NPN, low
power, high frequency switch
.

B
ipolar junction transistor

The
bipolar junc
tion transistor

(BJT) was the first type of transistor to be mass
-
produced. Bipolar transistors are so named because they conduct by using both majority
and minority carriers. The three terminals of the BJT are named
emitter
,
base

and
collector
. Two
p
-
n ju
nctions

exist inside a BJT: the
base/emitter junction

and
base/collector junction
. "The [BJT] is useful in amplifiers because the currents at the
emitter and collector are controllable by the relatively small base current."
[9]

In an NPN
transistor operating in the active region, the emitter
-
base junction is forward biased, and
electrons are injected into the base region. Because the base is narrow,

most of these
electrons will diffuse into the reverse
-
biased base
-
collector junction and be swept into the
collector; perhaps one
-
hundredth of the electrons will recombine in the base, which is the
dominant mechanism in the base current. By controlling th
e number of electrons that can
leave the base, the number of electrons entering the collector can be controlled.
[9]

Unlike the FET, the BJT is a
low

input
-
impedance device. Also, as the base

emitter
voltage (
V
be
) is increased the base

emitter current and hence the collector

emitter current
(
I
ce
) increase exponentially according to the
Shockley diode model

and the
Ebers
-
Moll
model
. Because of this e
xponential relationship, the BJT has a higher
transconductance

than the FET.

Bipolar transistors can be made to conduct by exposure to light, since absorption of
photons in

the base region generates a photocurrent that acts as a base current; the
collector current is approximately beta times the photocurrent. Devices designed for this
purpose have a transparent window in the package and are called
phototransistors
.

Field
-
eff
ect transistor

The
field
-
effect transistor

(FET), sometimes called a
unipolar transistor
, uses either
electrons (in
N
-
channel FET
) or holes (in
P
-
channel FET
) for conduction. The four
terminals of the FET are named
source
,
gate
,
drain
, and
body

(
substrate
)
. On most FETs,
the body is connected to the source inside the package, and this will be assumed for the
following description.

In FETs, the drain
-
to
-
source current flows via a conducting channel that connects the
source

region to the
drain

region. The con
ductivity is varied by the electric field that is
produced when a voltage is applied between the gate and source terminals; hence the
current flowing between the drain and source is controlled by the voltage applied
between the gate and source. As the gate

source voltage (
V
gs
) is increased, the drain

source current (
I
ds
) increases exponentially for
V
gs

below threshold, and then at a roughly
quadratic rate





(where V
T

is the

threshold voltage at which drain current begins)
[10
]

in the "
space
-
charge
-
limited
" region above threshold. A quadratic behavior is not observed in modern devices,
for example, at the
65 nm

technology node.
[11]

For low noise at narrow
bandwidth

the higher input resistance of the FET is
advantageous.

FETs are divided into two families:
junction FET

(
JFET
) and
insulated gate FET

(IGFET). The IGFET is more commonly known as
metal

oxide

semiconductor FET

(
MOSFET
), from their original construction as a layer of metal (the gate), a layer of
oxide (the insulation), and a layer of semiconductor. Unlike IGFETs, the JFET gate forms
a PN
diode

with the channel which lies between the source and drain. Functionally,

this
makes the N
-
channel JFET the solid state equivalent of the vacuum tube
triode

which,
similarly, forms a diode between its
grid

and
cathode
. Also, both devices operate in the
depletion mode
, they both have a high input impedance, and they both conduct

current
under the control of an input voltage.

Metal

semiconductor FETs (MESFETs) are JFETs in which the
reverse biased PN
junction

is replaced by a metal

semiconductor
Schottky
-
junction. These, and the HEMTs
(high electron mobility transistors, or HFETs), in which a two
-
dimensional electron gas
with very high carrier mobility is used for charge transport, are especially suitable for use
at very high frequencies (microw
ave frequencies; several GHz).

Unlike bipolar transistors, FETs do not inherently amplify a photocurrent. Nevertheless,
there are ways to use them, especially JFETs, as light
-
sensitive devices, by exploiting the
photocurrents in channel

gate or channel

bod
y junctions.

FETs are further divided into
depletion
-
mode

and
enhancement
-
mode

types, depending
on whether the channel is turned on or off with zero gate
-
to
-
source voltage. For
enhancement mode, the channel is off at zero bias, and a gate potential can "en
hance" the
conduction. For depletion mode, the channel is on at zero bias, and a gate potential (of
the opposite polarity) can "deplete" the channel, reducing conduction. For either mode, a
more positive gate voltage corresponds to a higher current for N
-
c
hannel devices and a
lower current for P
-
channel devices. Nearly all JFETs are depletion
-
mode as the diode
junctions would forward bias and conduct if they were enhancement mode devices; most
IGFETs are enhancement
-
mode types.

Semiconductor material

The fi
rst BJTs were made from
germanium

(Ge).
Silicon

(
Si
) types currently predominate
but certain advanced microwave and high performance versions now employ the
compound semiconductor

material
gallium ars
enide

(
GaAs
) and the
semiconductor
alloy

silicon germanium

(
SiGe
). Single element semiconductor material (Ge and Si) is
described as
ele
mental
.

Rough parameters for the most common semiconductor materials used to make
transistors are given in the table below; it must be noted that these parameters will vary
with increase in temperature, electric field, impurity level, strain and various ot
her
factors:

Semiconductor material characteristics

Semiconductor

material

Junction
forward

voltage

V @ 25 °C

Electron
mobility

m²/(V∙s) @ 25
°C

Hole mobility

m²/(V∙s) @ 25
°C

Max. junction
temp.

°C

Ge

0.27

0.39

0.19

70 to 100

Si

0.71

0.14

0.05

150 to 2
00

GaAs

1.03

0.85

0.05

150 to 200

Al
-
Si junction

0.3





150 to 200

The
junction forward voltage

is the voltage applied to the emitter
-
base junction of a BJT
in order to make the base conduct a specified current. The current increases exponentially
as t
he junction
's

forward voltage is increased. The values given in the table are typical for
a current of 1 mA (the same values apply to semiconductor diodes). The lower the
junction forward voltage the better, as this means that less power is required to "dr
ive"
the transistor. The junction forward voltage for a given current decreases with increase in
temperature. For a typical silicon junction the change is approximately −2.1 mV/°C.
[12]

The density of mobile carriers in the channel of a MOSFET is a function of the electric
field forming the channel and of various other phenomena such as the impurity level in
the channel. Some impurities, called dopants, a
re introduced deliberately in making a
MOSFET, to control the MOSFET electrical behavior.

The
electron mobility

and
hole mobility

columns show the average speed that electrons
and holes diffuse through the semiconductor material with an
electric field

of 1

volt per
meter applied across the material. In general, the higher the electron mobility
,

the faster
the transistor. The table indicates that Ge is a better material than Si in this respect.
However, Ge has four major shortcomings compared to silicon and
gallium arsenide:



its maximum temperature is limited



it has relatively high
leakage current




it cannot withstand high voltages



it is less suitable for fabricating integrated circuits

Because the electron mobility is higher than the hole mobility for all

semiconductor
materials, a given bipolar
NPN transistor

tends to be faster than an equivalent
PNP
transistor

type. GaAs has the highest electron mobility of the three semiconductors. It is
for this reason that GaAs is used in high frequency applications.
A relatively recent FET
development, the
high electron mobility transistor

(
HEMT
), has a
heterostructure

(junction between different semiconductor materials) of aluminium galli
um arsenide
(AlGaAs)
-
gallium arsenide (GaAs) which has double the electron mobility of a GaAs
-
metal barrier junction. Because of their high speed and low noise, HEMTs are used in
satellite receivers working at frequencies around 12

GHz.

Max. junction tempe
rature

values represent a cross section taken from various
manufacturers' data sheets. This temperature should not be exceeded or the transistor may
be damaged.

Al
-
Si junction

refers to the high
-
speed (aluminum
-
silicon) semiconductor
-
metal barrier
diode, c
ommonly known as a
Schottky diode
.

This is included in the table because some
silicon power IGFETs have a
parasitic

reverse Schottky diode formed between the
source and drain a
s part of the fabrication process. This diode can be a nuisance, but
sometimes it is used in the circuit.

Packaging

Transistors come in many different packages

(
chip carriers
) (see images). The two main

categories are through
-
hole (or leaded), and

surface
-
mount, also known as surface mount

device (SMD). The ball grid array (BGA) is the

latest surface mount package (currently only for

large transistor array
s). It has solder "balls" on

the underside in place of leads. Because they are

smaller and have shorter interconnections, SMDs


have better high frequency characteristics but


lower power rating.

Transistor packages are made of glass, metal, ceramic or p
lastic. The package often
dictates the power rating and frequency characteristics. Power transistors have large
packages that can be clamped to
heat sinks

for enhanced cooling. Additionally, most
power transistors have the collector or drain physically con
nected to the metal can/metal
plate. At the other extreme, some surface
-
mount
microwave

transistors are as small as
grains of sand.

Often a given transistor type is available in different packages. Transistor packages are
mainly standardized, but the assig
nment of a transistor's functions to the terminals is not:
different transistor types can assign different functions to the package's terminals. Even
for the same transistor type the terminal assignment can vary (normally indicated by a
suffix letter to th
e part number
-

i.e. BC212L and BC212K).



Through
-
hole transistors

(tape measure marked in
centimeters
)

Working with Transistors:

---------------------------------------------------------------------------------------------------------

from the Transistor pages of the
The Electronics Club

website at:
www.kpsec.freeuk.com


with minor modifications. This site is generated by J. Hewes at
Kelsey Park Sports College.

------------------------------------------------------------------------------------------------------------

Tra
nsistor
Function
:

Transistors
amplify current
, for example they can be used to
amplify the small output current from a logic chip so that it can
operate a lamp, relay or other high current device. In many
circuits a resistor is used to convert the changing

current to a
changing voltage, so the transistor is being used to
amplify
voltage
.

A transistor may be used as a
switch

(either fully on with maximum current, or
fully off with no current) and as an
amplifier

(always partly on).

The amount of current am
plification is called the
current gain
, symbol
h
FE
.


Types of transistor
:

There are two types of standard
bipolar
transistors,
NPN

and
PNP
, with different circuit symbols. The
letters refer to the layers of semiconductor material
used to make the transistor. Most transistors used
today are NPN because this is the easiest type to
make from silicon. If you are new to electronics it is
best
to start by learning how to use NPN transistors.

The leads are labeled
base

(B),
collector

(C) and
e
mitter

(E).

These terms refer to the internal operation of a transistor but they are not much
help in understanding how a transistor is used, so just treat

them as labels!

A
Darlington

pair

i
s

two transistors connected together to give a very high
current gain.

In addition to standard (bipolar junction) transistors,
there are
field
-
effect
transistors

which are usually referred to as
FET
s. They have different circuit
symbols and properties and they are not covered
here
.



Transistor circuit
symbols

Connecting

Transistors:

Transistors have three leads which
must be connected the correct way
round. Please take care with this
because a wrongly connected
transistor may be damaged instantly
when you switch on.

If
you are lucky the orientation of the
transistor will be clear from the PCB
or
strip
board layout diagram,
otherwise you will need to refer to a
supplier's catalogue to identify the
leads.

The drawings on the right show the
leads for some of the most common

case styles.

Please note that transistor lead diagrams show the view from
below

with the
leads towards you. This is the opposite of IC (chip) pin diagrams which show the
view from above.


Soldering
:

Transistors can be damaged by heat when solderi
ng so if
you are not an expert it is wise to use a heat sink clipped
to the lead between the joint and the transistor body. A
standard crocodile clip can be used as a heat sink.



Heat sinks
:

Waste heat is produced in transistors due to the current flowing
through them. Heat sinks are needed for power transistors
because they pass large currents. If you find that a transistor is
becoming too hot t
o touch it certainly needs a heat sink! The heat
sink helps to dissipate (remove) the heat by transferring it to the
surrounding air.




Transi
stor leads for some common case styles.




Heat sink



Testing a transistor
:

Transistors can be damaged by heat when soldering or by misuse in a circuit. If
you suspect that

a transistor may be
damaged there are two easy ways to test it:

1. Testing with a multimeter

Use a
multimeter

or a
simple

tester

(battery,
resistor and LED) to check each pair of leads
for conduction. Set a digital multimeter to
diode test and an analogue mult
imeter to a
low resistance range.


Test each pair of leads

both ways
:



The
base
-
emitter (BE)

junction
should behave like a diode and
conduct one way only
.



The
base
-
collector (BC)

junction should behave like a diode and
conduct
one way only
.



The
collecto
r
-
emitter (CE)

should
not conduct either way
.

The diagram shows how the junctions behave in an NPN transistor. The diodes
are reversed in a PNP transistor but the same test procedure can be used.


2. Testing in a simple switching circuit

Connect the transistor into the circuit shown on
the right which uses the transistor as a switch.
The supply voltage is not crit
ical
. A
ny

voltage

between 5 and 12V is suitable. This circuit can be
quickly built on
a
breadboard

for example. Take
care to include the 10k

resistor in the base
connection or you will destroy the transistor as
you test it!

If the transistor is OK the LED should light when
the switch is pressed and not light when the
switch is released.

To test a PNP transistor use th
e same circuit but
r
everse the LED and the supply voltage.

Some
multimeters

have a

transistor
test


function which provides a known base current and measures the collector
current so as to display

the transistor's DC current gain h
FE
.



Testing an NPN transistor


A simple switching circuit

to test an NPN transistor

Transistor codes
:

There are three main series of transistor codes used in the UK:



Codes beginning with B (or A), for example BC108, BC478


The first letter B is for silicon, A is for germanium (rarely used now). Th
e second letter
indicates the type; for example C means low power audio frequency; D means high
power audio frequency; F means low power high frequency. The rest of the code
identifies the particular transistor. There is no obvious logic to the numbering s
ystem.
Sometimes a letter is added to the end (eg BC108C) to identify a special version of the
main type, for example a higher current gain or a different case style. If a project specifies
a higher gain version (BC108C) it must be used, but if the general

code is given (BC108)
any transistor with that code is suitable.




Codes beginning with TIP, for example TIP31A


TIP refers to the manufacturer: Texas Instruments Power transistor. Odd numbers are
NPN, even numbers are PNP. The letter at the end identifies

versions with different
voltage ratings.




Codes beginning with 2N, for example 2N3053


The initial '2N' identifies the part as a transistor and the rest of the code identifies the
particular transistor. There is no obvious logic to the numbering system.



Choosing a transistor:


Most projects will specify a particular transistor, but if necessary you can usually
substitute an equivalent transistor from the wide range available. The most
important properties to look for are the maximum collector current I
C

and the
current gain h
FE
. To make selection easier most suppliers group their transistors
in categories determined either by their
typical use

or
maximum power

rating.

To make a final choice you will need to consult the tables of technical data which
are

normally provided in catalogues. They contain a great deal of useful
information but they can be difficult to understand if you are not familiar with the
abbreviations used. The table below shows the most important technical data for
some popular transist
ors, tables in catalogues and reference books will usually
show additional information but this is unlikely to be useful unless you are
experienced. The table rates the transistors based on the following properties.

Structure:
This refers to the type of t
ransistor, NPN or PNP. The polarities of
the two type are different, so if you are looking for a substitute, it must be the
same type.

Case Style:


This refers to the style of case the transistors is available in. Refer
to the cases shown in the Connecti
ng section or see the manufacturers catalog.

I
C

max:

The maximum collector current

V
CE

max:

The maximum voltage across the collector
-
emitter junction. You can
ignore this rating in low voltage circuits.

h
FE

:
This is the current gain (strictly the DC cu
rrent gain). The guaranteed
minimum value is given because the actual value varies from transistor to
transistor

even for those of the same type! Note that current gain is just a ratio,
so it has no units. The gain is often quoted at a particular collec
tor current I
C

which is usually in the middle of the transistor’s range. Sometimes minimum and
maximum value are given. Since the gain is roughly constant for various
currents but it varies from transistor to transistor, this detail is only really of
int
erest to experts.

P
tot

max:

Maximum total power which can be developed in a transistor. Note
that a heat sink will be required to achieve the maximum rating. This rating is
important for transistors operating as amplifiers, the power is roughly I
C

x

V
CE

.
For transistors operating as switches, the maximum collector current (I
C

max,) is
more important.

Category:

This shows the typical use for the transistor, it is a good starting point
when looking for a substitute. Catalogs may have separate tabl
e for different
categories.

Possible substitutes:

These are transistors with similar electrical properties
which will be suitable substitutes in most circuits. However, they may have a
different case style so you will need to take care when placing them

on the circuit
board.


Darlington pair:


This is two transistors connected together so that the amplified current from the
first is amplified further by the second transistor. This gives the Darlington pair a
very high current gain such as 10000. Darlingt
on pairs are sold as complete
packages containing the two transistors. They have three leads (
B
,

C

and

E
)
which are equivalent to the leads of a standard individual transistor.

You can make up your own Darlington pair from two
transistors.

For example:



For TR1 use BC548B with h
FE1

= 220.



For TR2 use BC639 with h
FE2

= 40.

The overall gain of this pair is h
FE1

× h
FE2

= 220 × 40 = 8800.

The pair's maximum collector current I
C
(max) is the same as TR2.


Common Bipolar Transistors:

NPN transistors

Code

St
ructure

Case

style

IC

max.

VCE

max.

hFE

min.

Ptot

max.

Category

(typical use)

Possible

substitutes

BC107

NPN

TO18

100mA

45V

110

300mW

Audio, low power

BC182 BC547

BC108

NPN

TO18

100mA

20V

110

300mW

General purpose, low power

BC108C BC183
BC548

BC108C

NP
N

TO18

100mA

20V

420

600mW

General purpose, low power



BC109

NPN

TO18

200mA

20V

200

300mW

Audio (low noise), low power

BC184 BC549

BC182

NPN

TO92C

100mA

50V

100

350mW

General purpose, low power

BC107 BC182L

BC182L

NPN

TO92A

100mA

50V

100

350mW

General
purpose, low power

BC107 BC182

BC547B

NPN

TO92C

100mA

45V

200

500mW

Audio, low power

BC107B

BC548B

NPN

TO92C

100mA

30V

220

500mW

General purpose, low power

BC108B

BC549B

NPN

TO92C

100mA

30V

240

625mW

Audio (low noise), low power

BC109

2N3053

NPN

TO39

7
00mA

40V

50

500mW

General purpose, low power

BFY51

BFY51

NPN

TO39

1A

30V

40

800mW

General purpose, medium
power

BC639

BC639

NPN

TO92A

1A

80V

40

800mW

General purpose, medium
power

BFY51

TIP29A

NPN

TO220

1A

60V

40

30W

General purpose, high power



TIP31
A

NPN

TO220

3A

60V

10

40W

General purpose, high power

TIP31C TIP41A

TIP31C

NPN

TO220

3A

100V

10

40W

General purpose, high power

TIP31A TIP41A

TIP41A

NPN

TO220

6A

60V

15

65W

General purpose, high power



2N3055

NPN

TO3

15A

60V

20

117W

General purpose, hi
gh power




PNP transistors

Code

Structure

Case

style

IC

max.

VCE

max.

hFE

min.

Ptot

max.

Category

(typical use)

Possible

substitutes

BC177

PNP

TO18

100mA

45V

125

300mW

Audio, low power

BC477

BC178

PNP

TO18

200mA

25V

120

600mW

General purpose, low pow
er

BC478

BC179

PNP

TO18

200mA

20V

180

600mW

Audio (low noise), low power



BC477

PNP

TO18

150mA

80V

125

360mW

Audio, low power

BC177

BC478

PNP

TO18

150mA

40V

125

360mW

General purpose, low power

BC178

TIP32A

PNP

TO220

3A

60V

25

40W

General purpose, hig
h power

TIP32C

TIP32C

PNP

TO220

3A

100V

10

40W

General purpose, high power

TIP32A

Note: T
he data in this table was compiled from several sources which are not entirely consistent!




Transistor Circuits:


There are two types of standard transistors, NPN and
PNP, with different circuit symbols. The letters refer to
the layers of semiconductor material used to make the
transistor. Most
transistors used today are NPN
because this is the easiest type to make from silicon.
This page is mostly about NPN transistors and if you
are new to electronics it is best to start by learning
how to use these first.



Transistor currents

The diagram s
hows the two current paths through a transistor.
You can build this circuit with two standard 5mm red LEDs and
any general purpose low power NPN transistor (BC108, BC182
or BC548 for example).

The small
base current

controls the larger
collector
current
.


When the switch is closed

a small current flows into the
base (B) of the transistor. It is just enough to make LED B
glow dimly. The transistor amplifies this small current to
allow a larger current to flow through from its collector (C)
to its emitter (
E). This collector current is large enough to
make LED C light brightly.

When the switch is open

no base current flows, so the
transistor switches off the collector current. Both LEDs are
off.

A transistor amplifies current and can be used as a switch.


This arrangement where the emitter (E) is in the controlling
circuit (base current) and in the controlled circuit (collector
current) is called
common emitter mode
. It is the most
widely used arrangement for transistors so it is the one to
learn first.


Transistor circuit symbols

Fu
nctional model of an NPN transistor
:

The operation of a transistor is difficult to explain and understand in terms of its
internal structure. It is more helpful to use this functional model:



The base
-
emitter junction behaves like a
diode
.



A base current I
B

flows only when the voltage V
BE

across the base
-
emitter
junction is 0.7V or more.



The small base current I
B

controls the large collector current Ic.



Ic = h
FE

× I
B



(unless the transisto
r is full on and saturated)


h
FE

is the current gain (strictly the DC current gain), a typical value for h
FE

is 100 (it has no units because it is a ratio)



The collector
-
emitter resistance R
CE

is controlled by the base current I
B
:

o

I
B

= 0


R
CE

= infinity



transistor off

o

I
B

small


R
CE

reduced


transistor partly on

o

I
B

increased


R
CE

= 0


transistor full on ('saturated')


Additional notes:



A resistor is often needed in series with the base connection to limit the
base current I
B

and prevent the tran
sistor being damaged.



Transistors have a maximum collector current Ic rating.



The
current gain h
FE

can vary widely
, even for transistors of the same
type!



A transistor that is
full on

(with R
CE

= 0) is said to be '
saturated
'.



When a transistor is satur
ated the collector
-
emitter voltage V
CE

is reduced
to almost 0V.



When a transistor is saturated the collector current Ic is determined by the
supply voltage and the external resistance in the collector circuit, not by
the transistor's current gain. As a re
sult the ratio Ic/I
B

for a saturated
transistor is less than the current gain h
FE
.



The emitter current I
E

= Ic + I
B
, but Ic is much larger than I
B
, so roughly



I
E

= Ic.





Darlington pair
:

This is two transistors connected together so that
the current amplified by the first is amplified further
by the s
econd transistor. The overall current gain is
equal to the two individual gains multiplied
together:

Darlington pair current gain, h
FE

= h
FE1

× h
FE2


(h
FE1

and h
FE2

are the gains of the individual
transistors)

This gives the Darlington pair a very high c
urrent
gain, such as 10000, so that only a tiny base
current is required to make the pair switch on.

A Darlington pair behaves like a single
transistor with a very high current gain.

It has
three leads (
B
,

C

and

E
) which are equivalent to
the leads of a s
tandard individual transistor. To
turn on there must be 0.7V across both the base
-
emitter junctions which are connected in series
inside the Darlington pair, therefore it requires 1.4V
to turn on.

Darlington pairs are available as complete packages but yo
u can make up your
own from two transistors; TR1 can be a low power type, but normally TR2 will
need to be high power. The maximum collector current Ic(max) for the pair is the
same as Ic(max) for TR2.

A Darlington pair is sufficiently sensitive to respon
d to the small current passed
by your skin and it can be used to make a
touch
-
switch

as shown in the
diagram. For this circuit which just lights an LED the two transistors can be any
general purpose low power transistors. The 100k

resistor protects the
transistors if the contacts are linked with a piece of wire.









Touch switch circuit

Using a transistor as a switch
:

When a transistor is used as a switch it must be
either
OFF

or
fully ON
. I
n the fully ON state the
voltage V
CE

across the transistor is almost zero
and the transistor is said to be
saturated

because
it cannot pass any more collector current Ic. The
output device switched by the transistor is usually
called the 'load'.


The powe
r developed in a switching transistor is
very small:


--

In the OFF state: power = Ic × VCE, but Ic = 0, so the power is zero.


--

In the full ON state: power = Ic × VCE, but VCE = 0 (almost), so the


power is very sm
all.

This means that the transistor should not become hot in use and you do not need
to consider its maximum power rating. The important ratings in switching circuits
are the
maximum collector current Ic(max)

and the
minimum current gain
h
FE
(min)
. The tra
nsistor's voltage ratings may be ignored unless you are using a
supply voltage of more than about 15V.


Protection diode
:

If the load is a
motor
,
relay

or
solenoid

(or
any other device with a coil) a
diode

must be
connected across the load to protect the
transistor (and chip) from damage when the
load is switched off. The diagram shows how
this is connected 'backwards' so that it will
normally NOT conduct. Conduction only
occurs when the loa
d is switched off, at this
moment current tries to continue flowing
through the coil and it is harmlessly diverted
through the diode. Without the diode no
current could flow and the coil would produce
a damaging high voltage 'spike' in its attempt
to keep
the current flowing.






When to use a
relay
:


Transistors cannot switch AC or high voltages and they are not usually a good
choice for switching large currents (>

5A). In these cases a
relay

will be needed,
but note that a low power transistor may still be
needed to switch the current for
the relay's coil!


Advantages of relays:



Relays can switch AC and DC, transistors

can only switch DC.


Relays can switch high voltages, transistors cannot.


Relays are a better choice for switching large currents (>

5A).


Relays can switch many contacts at once.


Disadvantages of relays:


Relays are bulkier than transistor
s for switching small currents.


Relays cannot switch rapidly, transistors can switch many times per second.


Relays use more power due to the current flowing through their coil.


Relays require more current than many chips can provide, so a low p
ower


transistor may be needed to switch the current for the relay's coil.


Connecting a transistor to the output from a chip
:

Most chips cannot supply large output currents so it may be necessary to use a
transistor to switch the larger curr
ent required for output devices such as lamps,
motors and relays. The 555 timer chip is unusual because it can supply a
relatively large current of up to 200mA which is sufficient for some output devices
such as low current lamps, buzzers and many relay co
ils without needing to use
a transistor.

A transistor can also be used to enable a chip connected to a low voltage supply
(such as 5V) to switch the current for an output device with a separate higher
voltage supply (such as 12V). The two power supplies m
ust be linked, normally
this is done by linking their 0V connections. In this case you should use an NPN
transistor.

A resistor R
B

is required to limit the current flowing into the base of the transistor
and prevent it being damaged. However, R
B

must be s
ufficiently low to ensure
that the transistor is thoroughly saturated to prevent it overheating, this is
particularly important if the transistor is switching a large current (> 100mA). A
safe rule is to make the base current I
B

about five times larger tha
n the value
which should just saturate the transistor.




Choosing a suitable NPN transistor

The circuit diagram shows how to connect an
NPN transistor
, this will switch on
the load when the chip output is
high
. If you need the opposite action, with the
loa
d switched on when the chip output is
low

(0V) please see the circuit for a
PNP

transistor

below.

The procedure below explains how to
choose a suitable switching transistor.

1.

The transistor's maximum collector
current Ic(max) must be greater than
the load current Ic.


load current Ic =


supply voltage
Vs

load resistance R
L

2.

The transistor's minimum current gain
h
FE
(min) must be at least
five

times
the load current Ic divided by the
maximum output current from the
chip.


h
FE
(min)

>


5 ×




load current Ic


max. chip current

3.

Choose a transistor

w
hich meets
these requirements and make a note of its properties:
Ic(max)

and
h
FE
(min)
.

You can find these properties for popular
transistors on the
table that was
presented earlier in these notes or from the transistor’s datasheet.

4.

C
alculate an approximat
e value for the base resistor:


R
B

=


Vc × h
FE


where

Vc

=

chip

supply

voltage


(in

a

simple

circuit

with

one

supply

this

is

Vs)

5 × Ic

5.

For a simple circuit where the chip and the load share the same power
supply (Vc

=

Vs) you may prefer to use: R
B

=

0.2

×

R
L

×

h
FE


6.

Then choose the nearest standard value for the base resistor.


7.

Finally, remember that if the load is a motor or relay coil a
protection diode

is required.



NPN transistor switch

(load is on when chip output is high)




Using units in calculations

Remember to use V, A and
or

V, mA and k
.

Example

The output from a 4000 series CMOS chip is required to operate a relay
with a
100

coil.

The supply voltage is 6V for both the chip and load. The chip can
supply a maximum current of 5mA.

1.

Load current = V
s
/R
L

= 6/100 = 0.06A = 60mA, so transi
stor must have
Ic(max) > 60mA
.

2.

The maximum current from the chip is 5mA, so transistor must have
h
FE
(min) > 60

(5

×

60mA/5mA).

3.

Choose general purpose low power transistor
BC182

with
Ic(max)

=

100mA

and
h
FE
(min)

=

100
.

4.

R
B

= 0.2 × R
L

× h
FE

= 0.2 × 100 × 1
00 = 2000
. so choose
R
B

=

1
.8kΩ
or
2.2 kΩ.

5.

The relay coil requires a
protection diode
.


Choosing a suitable PNP transistor:


The circuit diagram shows how to connect a
PNP transistor
, this will switch on the load
when the chip output is
low

(0V). If you need
the opposite act
ion, with the load switched
on when the chip output is
high

please see
the circuit for an
NPN

transistor

above.

The procedure for choosing a suitable PNP
transistor is exactly the same

as that for an
NPN transistor described above.


Using a transistor switch with sensors
:

The top circuit diagram shows an
LDR

(light sensor)
connected so that the LED lights when th
e LDR is in
darkness. The variable resistor adjusts the brightness at
which the transistor switches on and off. Any general
purpose low power transistor can be used in this circuit.

The 10k

fixed resistor protects the transistor from
excessive base current (which will destroy it) when the
variable resistor is reduced to zero. To make this circuit
switch at a suitable brightness you may need to
experiment with different valu
es for the fixed resistor, but
it must not be less than 1k
.



PNP transistor switch

(load is on when chip output is low)

LED lights when the LDR is
dark



If the transistor is switching a load with a coil, such as a motor or relay,
remember to add a
protection

diode

across the load.

The switching action can be inverted
, so the LED lights when the LDR is
brightly lit, by sw
a
pping the LDR and variable resistor. In this case the fixed
resistor c
an be omitted because the LDR resistance
cannot be reduced to zero.

Note that the switching action of this circuit is not
particularly good because there will be an
intermediate brightness when the transistor will be
partly on

(not saturated). In this sta
te the transistor is
in danger of overheating unless it is switching a small
current. There is no problem with the small LED
current, but the larger current for a lamp, motor or
relay is likely to cause overheating.

Other sensors, such as a
thermistor
,

can be used with this circuit, but they may
require a different variable resistor. You can calculate an approximate value for
the variable resistor (R
v
) by using a
multimeter

to find the minimum and
maximum values of the sensor's resistance (R
min

and R
max
):




Variable resistor
:


You can make a much better switching circuit w
ith sensors connected to a
suitable IC (chip). The switching action will be much sharper with no partly on
state.


A transistor inverter (NOT gate)
:

Inverters (NOT gates) are available on logic chips but if you
only require one inverter it is usually bet
ter to use this circuit.
The output signal (voltage) is the inverse of the input signal:



When the input is high (+V
s
) the output is low (0V).



When the input is low (0V) the output is high (+V
s
).

Any general purpose low power NPN transistor can be used.
For general use
R
B

=

10k

and R
C

=

1k
, then the inverter output can be connected to a dev
ice
with an input impedance (resistance) of at least 10k

such as a logic chip or a
555 timer (trigger and reset inputs).

If you are connecting the inverter to a CMOS logic

chip input (very high
impedance) you can increase R
B

to 100k

and R
C

to 10k
, this will r
educe the
current used by the inverter.



LED lights when the LDR is
bright




Laboratory 08: Section B: Breadboarding Transistor Circuits


In this section you are to experiment with transistors and their ability to work as
electronically controlled switches.
The electrical components that a
re often used to
determine the control logic of an electrical device don't have enough power to drive very
large loads. Therefore their output is usually sent to a transistor circuit which provides
power to whatever device is controlled.



Step 1: Build

an input switch and Digital Inverter circuit:


You will start by setting up a circuit that will provide a digital signal
output signal which
can
toggle between
high and low states, but will only
be able to drive a
small output
current. This will be buil
t using a TTL level digital logic chip, the 740
4

hex inverter gate
chip.



Using a bread board, set up the circuit shown below.


When the switch is closed

the input to the inverter

is grounded or held at a low state.

The output then goes high.


When th
e switch is open,

the pull up resister holds the

input voltage to 5V, which

causes the output of the inverter

to be low.


What is the purpose of the

330Ω resister?


___________________________


__________________________


__________________________


Typi
cal LED set up a voltage drop of between 1.7 and 2.0 volts.

Assuming a voltage of 2.0 volts across the LED, use Ohm's Law to determine the current
that flows through the 330 ohm resister and LED.




If you replace the LED with a small DC motor, can the in
verter
ch
ip run the motor?
_____


Ω

Ω

Step 2: Build

a
n

NPN Bipolar Transistor Circuit
:


You will be adding a simple transistor switch circuit to the output of the switch controlled
inverter.


The NPN transistor turns on when current flows into the base of the
transistor. As
current flows through the transistor from base to the emitter, this turns on the

transistor
which allows the c
urrent to flow from the collector to the emitter.


The basic circuit
using an
NPN transistor is shown below.

The resistance shown
as R
Load

refers to the device you wish to drive.

The purpose of the resistor R
2

is to limit the

amount of current flowing into the base.

R
2

will often be in the range of 10 to 100 k
Ω.


When V
S

and V
in

are from the same source

a rule of thumb
that can be used to size a

resistor for
R
base

if given by




where R
load,


is the load resistance, and

h
fe
, is the transistor gain.


Notice that only a
small amount
of current into the

base
is needed to set up a
large
current flow through

the load
.



Using the components below, s
how how you will wire up the
NPN
transistor
to turn
on an LED when the output from the inverter gate is
high.















R=330Ω


7404

SW

LED

R=1 kΩ

R=10 kΩ

GND

+5V

C

E

B

NPN

2N3
904

Emitter


Collector

Base

R
load


NPN transistor

+V
S

R
b
ase

Solut
ion:



















Now use your breadboard to hook
up this circuit
and demonstrate that the LED
lights when t
he switch is closed.



___ I think I breadboarded the circuit shown above correctly.


___ I did…I did….I did.


___ LED lights when I close t
he switch and turns off when the switch is open.


Why is
powering the LED using the transistor b
etter than connecting the LED
directly to the inverter gate?
















R=330Ω

C

E

B

NPN


7404

SW

LED

R=1 kΩ

R=10 kΩ

+5V

GND

GND

+5V

GND

Step 3: Build a PNP Transistor Circuit:


The other flavor of BJT transistor
, the PN
P transistor,

can also be used to power devices.
The major difference
between the NPN and the
PNP transistor is that current is turned on
to flow through the transistor when the base is held low

instead of high
. When the base
voltage is low,
current flo
ws from the emitter into the base, and this turns on the ability of
current to flow from the emitter to the collector.


A typical PNP transistor circuit is shown below.


Once again it
'
s important to chose an

appropriate value for the base

resistance.

Too low a resistance

to the base can set up too large

a current that can burn out the

transistor.


Once again,
if using the same

voltage as the power supply

to both the base and the

load, then the base resistor,

R
base,
can be sized using






Use the 2N3906 transistor and set

up a circuit that

takes the
digital logic

inverter

chip output
to turn on the LED.


Draw the diagram of this circuit below.














Demonstrate the PNP transistor switch.


____ The
LED lights

up then the switch is

open

/

closed

. (circle one )

Collector

Emitter

Base


R
load



+V
s

PNP transistor

R
base

V
in

Step 4:
Determining
Transistor
C
haracteristics:


The two transistors you used so far are considered small signal transistors.

While they seem quite capable of powering an LED or
possibl
y a

little

l
arger load,

they
may not be able to power devices which require larger amounts of current or power.


Locate the Datasheets on your course CD for both the 2N3904 and 2N3906 transistors.

When
choosing
transistors
to use as
switches, the most important
param
eters will include:

Transistor Type

(NPN or PNP),
Maximum Collector Current
,
I
Cmax
, and
M
aximum
T
ransistor
G
ain
,
h
fe
.


Locate values on the datasheets for each of these transistor

characteristic
s and write them
down
in the table below:


2N3904

2N3906

Transistor
Type



Max Collector Current (mA)



Max Transistor
Gain




If
a
datasheet for your transistor was not available, you can use a feature
on many
multimeters to
determine:


--

Type: NPN or PNP transistor


--

Pin Definition: which pi
ns are the Emitter, Collector, and Base


--

Gain: an approximate value of
h
fe


Examine your multimeter and locate the round 8
-
pin socket that is labeled with
NPN/PNP. Turn the multimeter dial to the h
fe

setting. To use the transistor function on
the multimeter, you simply insert the three legs of the transistor, so that they are each in
separate sockets labeled E, C, and B entirely on either the PNP or NPN side of the socket.
If you have inserted the transistor in its correct orientation, the mul
timeter will typically
give a gain reading between 50 and 300

(which are typical gain values for a single stage

transistor
)
. If the transistor is
inserted
in an incorrect orientation, the gain reading
displayed will either be near 0 or will appear to be

out of range. There are four possible
ways to insert the transistor on each of the NPN and PNP sides, so you will need to insert
the transistor a maximum of 8 different ways to determine not only the transistor gain,
but also its type, and which legs are

the (E)mitter, (C)ollector, and (B)ase respectively.

An

incorrect orientation will not harm the transistor

or the multimeter
.


Use your multimeter to confirm the

gain values provided for the 2N3904

and 2N3906 transist
o
rs are what the

datasheet says th
ey are.



2N3904: datasheet gain: _______ measured gain: ________



2N3906: datasheet gain: _______ measured gain: ________


In many switching applications, a
similar transistor can be used to replace another
transist
or.
As long as the
load
current
r
emains smaller than the m
aximum collector
current
of the transistor, the transistor probably won't burn out.


To see if you have become proficient with the multimeter transistor function, you have
been p
rovided with another small signal transistor
to test
. It has the
marking
s

of
MPS2907A
.

I have not provided you with the datasheet for this transistor.


Can you use your
multimeter transistor function to determine
information about this
unknown transistor:







Type: __________



Gain: ________





Pins: ___ ____ _____



The multimeter does n
ot provide any information
about the maximum collector current flow.
Typically the size and case style are related to the
maximum collector current that a transistor can
handle. General rule of thumb: the larger the
transistor, the larger the maximum
collector current.



You have also been provide
d

with two somewhat
larger transistors.
Examine the TIP 120 and TIP125 transistors. These
are considered
general purpose, high power transistors and are packaged in th
e TO220 style package.
Just from the fact they are larger

than the small signal transistors
, you would expect that
they will have a larger maximum collector current.


Open up their datasheets and determine their characteristics:


TIP120

TIP125

Type



M
ax Collector Current (mA)



Gain




If you examine the internal circuit
structure of these transistors, you will
notice that they are a two stage transistor.

This type of structure is called a
Darlington transistor. Multiple
transistor
stages increas
e the overall gain associated
with the transistor circuit.




Step 5: Running a motor
with a
transistor
.


Each of the

TIP 120 and TIP 125
Darlington transistor
s
, has plenty of capacity to power a
small DC motor.


Replace the small signal transistor i
n
each of your previous NPN and PNP

circuits with the appropriate high
power Darlington transistor and wire
up the motor to the circuit. Make
sure you include a protection diode
across the motor to help suppress
voltage spikes that result when the
transi
stor is turned off.



If you examine
a

diode, you should
find one end has a band that runs around

the circumference. This band represents

the forward
bias
direction

of the diode.


Note: If you are not sure
which
direction is positive

for a diode, ask

your instructor.
Hooking up the

diode incorrectly, can result in a burned out transistor.
















Will you need to select a different

resistor for R
2
with the motor acting as the load?

(yes / no)


____ Demonstrate driving a Motor using the
TIP 120 transistor.


____ Demonstrate driving a Motor using the TIP 125 transistor.


forward

R=330Ω

C

E

B

NPN


7404

SW

LED

R=1 kΩ

R
2
= ?

+5V

GND

GND

+5V

GND

M

Step 6: Setting up a MOSFET transistor circuit:



JFET (Junction Field Effect Transistors) and MOSFET (Metal
-
Oxide
-
Semiconductor
Field Effect Transistors) both offer a
dvantages over Bipolar transistors. Instead of
having a base, emitter, and collector, FETs have pins labeled Gate, Source, and Drain.
Instead of being activated by current flow into the Base like a Bipolar transistor, FETs
are activated by changing th
e voltage level applied to the Gate. FETs have smaller
internal current flows a
nd

therefore have
less power loss than Bipolar transistors of
similar capacity.



Typical circuit diagrams for two available channel MOSFET are given below.


Small signal N
-
Ch
annel MOSFET

High Power N
-
channel MOSFET

















Modify your Bipolar transistor circuit to use
the small signal N
-
channel MOSFET
(2N7000) in the circuit with the motor as the load.


Is this circuit able to power the DC motor?
(Yes / No)


Next modify your Bipolar transistor circuit to use the high power N
-
channel MOSFET
(IRLZ
4
4N) in the circuit with the motor as the load.


Is this circuit able to power the DC motor? ( Yes / No )


Demonstrate this circuit to the instructor.


Was

there any difference in the operation of the small signal and the high power
MOSFET transistor that you noticed? If so, describe.



__________________________________________________________________



optional

optional

R
load

R
loa
d

Gate

Gate

Drain

Drain

Source

Source

Step 7: Transistor circuits where the logic and supply

voltage are different.


In th
e previous
transistor
circuits,
you
used a transistor because you needed more
current

to drive the motor

than the logic chip could provide
. Instead of
connecting the
motor
directly to the
NOT gate, you used a transistor so th
at the 5 volt power supply could drive
the motor. The NOT gate and 5 volt supply both provide 5 volts but the NOT gate can’t
supply much current.
T
he NOT gate only needed to turn the transistor on and off. This
time you’re going to use transistors becau
se you want more
voltage

to the load
!


We'll set up this circuit using the N
-
channel MOSFET circuit you just used.
First setup
the
circuit shown below where the swi
tch circuit control
s

the NOT gate
. For the load we
will be using a
12 volt incandescent lig
ht

bulb. As a first run, connect the light bulb to
the 5 volt source and note how bright the bulb shines when the switch is closed.





For an

N
-
channel MOSFET circuit running at 5 volts
, did
you notice that the light was weak? (Yes / No ).









optional

R
load

Gate

Drain

Source

Throughout all of the setups always use 5 volts for the NOT
gate and the switch. Later the FET will get a
9

volt supply

Even t
hough the light worked, it wasn't operating with its expected power. This time
change the supply voltage to the light to a
9
volt supply

(using a battery)
. You will keep
the voltage to the logic chip at 5 V. Repeat, on
ly
connect the
9

V supply to the li
ght.
DO
NOT connect the
positive pole of the 9

volt

battery to the
logic gate.

















In order for this to work, the two voltage sources must also have a common ground
connection.
Make sure the
Ground connection of the 9 volt battery is
connect
ed to the
ground for 5 volt

supply
.

Otherwise the two voltage levels won’t be relative to the same
base
value.



____ Demonstrate that you are able to power the 12 V light bulb using a
9 volt battery
source.


We can also use a different voltage source

with a BJT. In general, M
OSFETs are
eas
ier
to
work with then BJTs
since they are voltage controlled

transistors.

BJTs are a bit
harder to use
because they are
current controlled transistors. Controlling the current
requires you to correctly select a r
esistor to limit the current flow.

If

we wish to use a
BJT to control the load subjected to the
9

V source, you may need to resize the Base
resistor.
To determine the proper resistance


1) Determine the current required to flow through the load:





2) Size the Base Resistor using:




What is an appropriate resistance value to use with BJT in this circuit.



optional

optional

R
load

R
load

Gate

Gate

Drain

Drain

Source

Source

+
9

V

+
9

V


Replace the MOSFET in the circuit with the TI
P 120 BJT using
t
he base resistance
specified, set up the transistor circuit which will run provide power to the bulb using the
9 volt
supply.



____ Demonstrate that you are able to power the light using a 5 volt control signal and a
9
volt load supply.






Summary:


What should you have learned from this lab
?


1) Transistors can be used
as electrically controlled switch
es

which use small amounts of
electrical power to control larger amounts of electrical power.


2) There are two main types of transisto
rs: Bipolar Junction and Field Effect Transistors


BJTs are controlled by the amount of current the runs into or out of the Base.


FETs are controlled by the voltage applied to the Gate.


3) There are two varieties of each type: NPN and PNP BJ
T


NPN BJTs turn on when the current flows into the Base.


PNP BJTs turn on when the current flows out of the Base.


4) The relative size of a transistor is related to the amount of current it can handle.


5) For transistors acting as switc
hes, the important transistor characteristics when sizing
the transistor are the Maximum Collector Current the transistor can handle and the
Maximum Transistor Gain.


6) Transistors can be used to power DC circuits at voltage
s

other than the logic chip
vo
ltage.


7) When powering devices
which use an inductance
coil
to set up an
electromagnetic
field, diodes should be used
across the input of the inductance device
to protect the
electronics form voltage spikes

when the switch closes
.

They effective act as

surge
protectors for more delicate electronics.


8) You should now know how to wire up simple transistors to drive s
mall
DC loads.



This concludes Optional Lab 08.