The transistor as a switch
Because a transistor's collector current
is proportionally limited by its base current, it can be used as a sort
of current-controlled switch. A relatively small flow of electrons sent
through the base of the transistor has the ability to exert control over
a much larger flow of electrons through the collector.
Suppose we had a lamp that we wanted to
turn on and off by means of a switch. Such a circuit would be extremely
simple:
For the sake of illustration, let's
insert a transistor in place of the switch to show how it can control
the flow of electrons through the lamp. Remember that the controlled
current through a transistor must go between collector and emitter.
Since it's the current through the lamp that we want to control, we must
position the collector and emitter of our transistor where the two
contacts of the switch are now. We must also make sure that the lamp's
current will move against the direction of the emitter arrow
symbol to ensure that the transistor's junction bias will be correct:
In this example I happened to choose an
NPN transistor. A PNP transistor could also have been chosen for the
job, and its application would look like this:
The choice between NPN and PNP is really
arbitrary. All that matters is that the proper current directions are
maintained for the sake of correct junction biasing (electron flow going
against the transistor symbol's arrow).
Going back to the NPN transistor in our
example circuit, we are faced with the need to add something more so
that we can have base current. Without a connection to the base wire of
the transistor, base current will be zero, and the transistor cannot
turn on, resulting in a lamp that is always off. Remember that for an
NPN transistor, base current must consist of electrons flowing from
emitter to base (against the emitter arrow symbol, just like the lamp
current). Perhaps the simplest thing to do would be to connect a switch
between the base and collector wires of the transistor like this:
If the switch is open, the base wire of
the transistor will be left "floating" (not connected to anything) and
there will be no current through it. In this state, the transistor is
said to be cutoff. If the switch is closed, however, electrons
will be able to flow from the emitter through to the base of the
transistor, through the switch and up to the left side of the lamp, back
to the positive side of the battery. This base current will enable a
much larger flow of electrons from the emitter through to the collector,
thus lighting up the lamp. In this state of maximum circuit current, the
transistor is said to be saturated.
Of course, it may seem pointless to use a
transistor in this capacity to control the lamp. After all, we're still
using a switch in the circuit, aren't we? If we're still using a switch
to control the lamp -- if only indirectly -- then what's the point of
having a transistor to control the current? Why not just go back to our
original circuit and use the switch directly to control the lamp
current?
There are a couple of points to be made
here, actually. First is the fact that when used in this manner, the
switch contacts need only handle what little base current is necessary
to turn the transistor on, while the transistor itself handles the
majority of the lamp's current. This may be an important advantage if
the switch has a low current rating: a small switch may be used to
control a relatively high-current load. Perhaps more importantly,
though, is the fact that the current-controlling behavior of the
transistor enables us to use something completely different to turn the
lamp on or off. Consider this example, where a solar cell is used to
control the transistor, which in turn controls the lamp:
Or, we could use a thermocouple to
provide the necessary base current to turn the transistor on:
Even a microphone of sufficient voltage
and current output could be used to turn the transistor on, provided its
output is rectified from AC to DC so that the emitter-base PN junction
within the transistor will always be forward-biased:
The point should be quite apparent by
now: any sufficient source of DC current may be used to turn the
transistor on, and that source of current need only be a fraction of the
amount of current needed to energize the lamp. Here we see the
transistor functioning not only as a switch, but as a true amplifier:
using a relatively low-power signal to control a relatively large
amount of power. Please note that the actual power for lighting up the
lamp comes from the battery to the right of the schematic. It is not as
though the small signal current from the solar cell, thermocouple, or
microphone is being magically transformed into a greater amount of
power. Rather, those small power sources are simply controlling
the battery's power to light up the lamp.
- REVIEW:
- Transistors may be used as switching
elements to control DC power to a load. The switched (controlled)
current goes between emitter and collector, while the controlling
current goes between emitter and base.
- When a transistor has zero current
through it, it is said to be in a state of cutoff (fully
nonconducting).
- When a transistor has maximum current
through it, it is said to be in a state of saturation (fully
conducting).
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