Elenco Electronic Playground 50-in-1 Experiments - Manual (Page 26)

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
Electronic Playground, 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.
INTRODUCTION TO TRANSISTORS
-26-
Transistor
Symbol for
NPN Transistor
Emitter
Collector
Base
EBC