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Electronics - Devices

Nov 2, 2013 (4 years and 7 months ago)

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Transistors

Physicists John Bardeen, Walter Brattain, and
William Shockley built the first transistor in 1947 at
Bell Telephone Laboratories. They called it a
transistor because it was said to tr
ansfer resistance.
Their invention was arguably the most important of
the twentieth century as it led directly to the electronic
revolution that we benefit from today. Personal
computers, mobile phones, game stations, digital
watches, CD players and a host

of other everyday
devices would not be possible without Bardeen, Brattain and Shockley’s discovery.

Transistors are mostly used for one of two purposes. They can be employed as an
electrical switch or more importantly as an amplifier, which is a device
that allows a
small electric current to control a much larger one.
Lets briefly go back to the variable resistor, by
turning the knob we varied the resistance in a
circuit. Now lets imagine a variable resistor that can
change its resistance electronically
without needing
to turn the knob. By supplying a small signal to this
device we can control how much resistance is in a
circuit. If we control resistance in a circuit we are
also controlling current (have another look at our
variable resistor controlled LE
D if this does not
make sense). A transistor can be thought of as an
electronically controlled variable resistor.

Most transistors have a small plastic case and three
metal legs sticking out from underneath. Each leg
has particular functions that are name
d collector, base and emitter. These names are
derived from thermionic valves (vacuum tubes) that preceded transistors. Some power
transistors have only two legs with its metal case filling the third legs role. We can
relate the functions of each leg by th
inking back to the variable resistor analogy used
in the preceding paragraph. We could think of the emitter and collector as being
equivalent to the resistive track in a variable resistor and the base as an electronic
adjusting knob. The base would be used

to electronically control current flowing
between emitter and collector.

Lets use a variable resistor and a transistor to control an electric motors speed. The
electric motor is fairly large and
requires a strong battery to power it.
There are two circu
its to look at
when designing transistor
-
controlled
devices. Lets call them the load and
control circuits. Firstly we shall look
at the load circuit (or collector

emitter circuit to give its proper
name). If the motor were to be run at