The document discusses the transistor, the basic building block of electronics. It describes the two main types - bipolar junction transistors and field effect transistors. Transistors can be used as amplifiers or switches to increase signal amplitude or turn devices on/off. Characteristics like packaging, markings, and applications of small signal and power transistors are covered. Circuit examples show how transistors function as amplifiers and switches.

1. Components - The Transistor
Transistors have many applications in electronics as discrete devices or
within integrated circuits. Transistors are Active components since
they provide gain.
There are two primary types of transistor – The Bipolar (Junction)
Transistor and the Unipolar or Field Effect Transistor (FET).
The transistor can be used either as an amplifier or as a switch, for example to
increase the amplitude of a signal or switch on an LED.
Example Circuit Symbol (BJT)
VCC +
Gnd (0V)
Input signal
(sinewave)
Output signal
(sinewave)
VCC +
Gnd (0V)
R
LED
Input signal
(pulse)

2. Transistor manufactures use an alphanumeric code printed on the device as
identification e.g. BC108, 2N3055, TIP31.
It is necessary to refer to manufacturers data sheets to obtain lead information and
ratings.
The case style and packaging varies from miniature plastic TO92 or metal can
TO18 for general purpose small signal types to TO220 and TO3 metal can
for high power applications.
Transistor Markings and Packaging
TO-220 TO-3TO-92 TO-18

3. Small Signal Transistors
Small signal transistors are used in circuits working on low signal amplitudes, high
frequency etc and requiring high current gain eg pre-amplifiers, radio, display
control. Most can operate up to 200mA providing the junction temperature is
not greater than 75o
C.
Small signal transistors cannot disperse their heat due to the small case size and
therefore some designs may require a heat sink to maintain a safe operating
temperature.

4. Power Transistors
In some cases it is necessary to pass a high current through a transistor in order to
drive a load. This can result in heat being generated in the component.
We must ensure that the component continues to operate to specification and no
damage is caused as a result of this current.
The heat generated is also known as power dissipation.
Power transistors are designed to be mounted on a heat sink to remove heat
generated and maintain a safe operating temperature.

5. Bipolar (Junction) Transistor
Consists of two p-n junctions back-to-back in the same semiconductor.
There are two possible arrangements n-p-n or p-n-p where a p-type or n-type
semiconductor material called the base is contained between two thicker slices
of the opposite material.
p
E
C
n
n
B
Collector
Emitter
Base
NPN
B
C
E
PNP
n
C
E
P
P
B
C
E
B
Collector
Emitter
Base
Bipolar transistors have three terminals base, collector and emitter.
The emitter always has an arrow indicating the direction of conventional current
flow, (electron flow is opposite).

6. Function of a Transistor
The bipolar transistor is used to amplify current
A small input current is applied to the base terminal causing a much larger current
to flow in the collector/emitter section.
The amount of amplification is called the current gain and is known as the hFE and
can range from 20 in power transistor to 400 in small signal transistors.
Current Flow
(conventional current)
IC
IB
IC + IB
IC
IB
hFE = or IC = hFE IB

7. The Darlington Pair
Connecting a small signal transistor in series with a power transistor we can
obtain a much larger current gain. This type of transistor is known as a
Darlington or Darlington pair.
Darlington transistors are ideal for interfacing low power electronic
control circuits with motors, solenoids and relays. They are also used in
digital control applications such as washing machines, drinks machines,
dish washers.
IC
IB
hFE = = hFE1 x hFE2
TR1
IB
IC
TR2
C
B
E

8. Transistor Circuits
Activity
1. A transistor is stated as having a current gain of 150 what does this
mean, give an example.
2. A transistor with a hFE of 200 is required to illuminate a 12v, 60mA
lamp. Determine the base current required.
3. A 20µA current source is to be used with a transistor having an hFE
ranging from 60 to 200. What would be the maximum collector current
achievable for reliable operation.
4. It is required to drive a relay coil from a digital control circuit. If the
relay requires 1 amp to operate the contacts and the digital control
can only source 1mA state the required current gain and a most
suitable type of transistor to be used, giving reasons.

9. Transistor Ratings
Manufactures data sheets provide many details to enable the selection of the most
suitable device for a particular situation.
A selection of common parameters is shown below.
VCE is the maximum collector/emitter voltage
IC max current
PTOT max power dissipation
hFE minimum current gain
fT the maximum operating frequency
Type No Pol. Vce
(V) Ic
(max) PTOT
hFE
(min) fT (MHz) Case
BC109 npn 25 200mA 750mW 240 150 TO-18
BC179 pnp 25 200mA 300mW 240 200 TO-18
TIP122 npn 100 5A 65W 1000 1 TO-220
BD135 npn 45 1.5A 12.5W 40@1.5A 160 TO-126
BD136 pnp 45 1.5A 12.5W 40@1.5A 140 TO-126
TIP31A npn 60 3A 40W 10@1A 3 TO-220
TIP32A pnp 60 3A 40W 25@1A 3 TO-220
2N3055 npn 60 15A 117W 20@4A - TO-3
TIP2955 pnp 100 15A 90W 20@4A - TO-218

10. Single Stage Transistor Amplifier
(practical activity)
This task is to build and test a transistor amplifier.
You will gain experience in identifying and handling components, planning circuit
layout and use of test equipment.
circuit diagram
The activity also forms part of your achievement criteria.
VCC +12V
Gnd 0V
OUTIN
TR1
R2
R1
R3
R4 C3
C1
C2

11. Transistor Switch
(practical activity)
This task is to build and test a light detector circuit using the
transistor as a switch.
The activity will enhance your experience of component types and
their applications in electronics.
VCC +12V
Gnd 0V
LDR
R1
TR1
R2
Lamp
circuit diagram
The activity also forms part of your achievement criteria.