Elenco Basic Electronic Experiments - Manual (Page 27)

INTRODUCTION TO TRANSISTORS
The Transistor: The transistor was first developed in 1949 at Bell Telephone Laboratories, the name being derived from
“transfer resistor”. It has since transformed the world. Did you ever hear of something called a vacuum tube? They are
large and can be found in old electronic equipment and in museums. They are seldom used today and few engineers even
study them now. They were replaced by transistors, which are much smaller and more reliable.
The transistor is best described as a current amplifier - it uses a small amount of current to control a large amount of
current. There are many different families of transistors but we will only discuss the type included in your PK-101, called
the NPN Bipolar Junction Transistor or BJT and made of the semiconductor silicon. It has three connection points, called
the emitter, base, and collector.
In our water pipe analogy the BJT may be thought of as the lever pivot shown here:
Notice that it includes a check valve that is connected to a lever arm. A small amount of “base current” pushes on the check
valve which turns and opens the lever arm. But before this base current can start to flow though it must have enough water
pressure to overcome the spring in the check valve (usually 0.7V). If the base pipe is much smaller than the collector and
emitter pipes, then a small base current I
B
flowing in will cause a large collector current I
C
to flow in, these will combine
and exit the device as emitter current I
E
.
In transistors the emitter, base, and collector are different regions of permanent electrical charge, producing the effects
described above for the lever pivot. The properties and uses of transistors may seem confusing at first but will become
clear as you proceed through the experiments. All but one of the remaining experiments will use the transistor, so its
importance to electronics should be apparent.
A key advantage of semiconductors is that several transistors can be manufactured on a single piece of silicon. This led
to the development of Integrated Circuit (IC) technology, in which careful control of complex manufacturing processes has
enabled entire circuits consisting of transistors, diodes, resistors, and capacitors to be constructed on a silicon base. Some
ICs used in computers now have more than a million transistors on them. Spectacular improvements in cost, size, and
reliability have been achieved as a result.
The schematic symbol for a transistor is shown below:
Note the small arrow in the emitter, this indicates which direction the current will flow through the device.
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COLLECTOR
EMITTER
BASE
TRANSISTOR Symbol for NPN TRANSISTOR
Flat