- A transistor is an electronic component
that acts like a valve to control
the amount of current passing through it.
Much like how this valve on the faucet
controls the amount of water flowing through the hose.
A transistor has three terminals.
One terminal brings current into the transistor,
and a second terminal carries that current out.
The third terminal is like this handle,
and it's used to control the amount of current
passing through those other two terminals.
Now unlike this handle which I adjust by physically moving,
we control the current through a
transistor by using an electrical signal.
Based on that electrical signal,
I can have the transistor operating in one of three modes.
I can turn the transistor so that
the valve is completely open,
and the current passes through it more or less unrestricted.
This is known as the saturation mode,
and the transistor is basically acting like a short circuit.
On the other extreme,
I could close that valve all the way
to completely block the flow of
current through the transistor.
Now the transistor is operating
in the cut-off mode like an open circuit.
When I use a transistor to select
between one of these two modes,
either completely open or completely closed,
I can use the transistor to switch
on or off parts of a circuit.
And computers are built of millions
or even billions of tiny little transistors doing just that.
Switching on and off to represent
ones and zeroes of digital information.
But a transistor doesn't just have to be used
in either on or off modes.
It can also operate in the third mode
known as the active region.
When it's in the active region,
it's as if the handle is somewhere in the middle.
And in the active region small changes
to the amount of current at that input control terminal
make much larger changes to the amount of current
passing through the transistor.
This allows the transistor to be used as an amplifier.
To take small input signals,
and make them much larger output signals.
There are two main types of transistors
that you'll encounter:
Bipolar junction transistors, which are
referred to as BJTs,
and field-effect transistors, or FETs.
Although both types of transistors have three terminals,
those terminals are given different names
in BJTs and FETs because those two types
of transistors have different physical structures.
On a BJT, the terminals are called the base,
emitter, and collector, and on FETs
they're called the gate, source, and drain.
Conceptually though, both types
of transistors perform a similar function.
The base terminal on a BJT, and the gate terminal on a FET
act like the valve handle on my faucet.
They're used to control the flow of current
between the other two transistor terminals.
Now, the way that those two types of
transistors are controlled is different.
The amount of current that a BJT lets through
is controlled by the amount of
current into its base terminal.
Whereas FETs are controlled by voltage at the gate.
But from a high level they're both just
electrically controlled valves
that are used to regulate an external source of current.
Since transistors have the ability
to control the flow of current through them,
based on another electrical signal,
they fall into a category of devices
known as active components.
Which includes transistors as well as the more complex
integrated circuit components, like operational amplifiers,
which are built using transistors.
Active components require an
external source of power to operate.
And because they draw power from an external source,
they can be used to amplify a signal.
Which means they can produce an output signal
with more power in it than the original input signal.
The opposite of active components are a category
of devices called passive components,
which includes resistors, capacitors,
inductors, and even diodes.
Passive components cannot be controlled
by another electrical signal.
They only respond to the voltage
across them and the current through them.
But, unlike active components, passive components
can operate without an external power source.
Although it's possible to build passive circuits
containing only passive components
that provide a voltage gain,
meaning the increase or decrease of the voltage of a signal,
passive components cannot produce a power gain.
The power out of a passive circuit
will always be less than, or at the very best,
equal to the input power.
Almost all circuits will include some
combination of passive components,
but they need to have at least one active component
to be considered an active circuit.
According to some definitions,
for a circuit to be properly called electronic,
it has to contain active components
controlling the flow of electricity.
I tend to throw around the term electronic
somewhat generously to describe
both active and passive circuits.
But active circuits really are
what make electronics exciting.