Components - The TransistorTransistors 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)

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

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 75oC.

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.

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.

Bipolar (Junction) TransistorConsists 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

nB

Collector

Emitter

Base

NPN

B

C

E

PNP

n

C

E

P

PB

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).

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

The Darlington PairConnecting 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

Transistor CircuitsActivity

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.

Transistor RatingsManufactures 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 voltageIC max current

PTOT max power dissipationhFE 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

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

R1R3

R4 C3

C1

C2

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.