Page 431 - Robot Builder's Bonanza

400 COMMON ELECTRONIC COMPONENTS FOR ROBOTICS

in. Transistors are rated by a number of criteria, which are beyond the scope of this book.
None of these ratings are printed directly on the transistor.
Transistors have three- or four- wire leads. The leads in the typical three- lead transistor are
base, emitter, and collector. (A few transistors, most notably certain types of the field- effect
transistor— or FET, for short— have a fourth lead. None of the circuits in this book use this
type, which are not common anyway.)

NPN, PNP— TWO SIDES OF THE SAME COIN
Transistors can be either NPN or PNP devices. This nomenclature refers to the sandwiching
of semiconductor materials inside the device. You can’t tell the difference between an NPN
and a PNP transistor just by looking at them.
However, the difference is indicated in the catalog specifications sheet as well as
by the schematic symbol for the transistor. In an NPN device, the arrow is shown
leaving the transistor; in a PNP device, it’s the opposite. This differentiation helps
NPN PNP you to quickly tell whether you should use an NPN or a PNP type transistor for the
transistor transistor
circuit.
ENTER THE MOSFET

Some semiconductor devices look and act like transistors and are actually called transistors,
but in reality they use a different underlying technology. So far I’ve been talking about a class
of transistor called the bipolar junction transistor, or BJT. These are by far the most com-
mon. Another form of transistor is the MOSFET. This collection of alphabet soup stands for
metal- oxide semiconductor field- effect transistor. It’s often used in circuits that demand high
current and high precision.
D D Wouldn’t you know it, but just to complicate things, MOSFET transistors don’t
G G use the standard base- emitter- collector connections you just read about. Instead,
S S they call them gate, drain, and source connections. And note, too, that the sche-
N-channel P-channel
MOSFET MOSFET matic diagram for the MOSFET is different from that of the standard transistor.
Like the bipolar junction transistor, MOSFETs come in two varieties: N- channel and
P- channel. And, as before, you can’t tell the difference between an N- channel and a P- channel
MOSFET just by looking at it.

The schematic symbols for the two types of MOSFETs are only ever- so- slightly different, and not
all schematic diagrams bother to show which one is used in the circuit. You cannot substitute
N- channel for P- channel, so when using these devices, be absolutely sure you’ve got the right
ones in your hands.

Integrated Circuits

The integrated circuit, colloquially referred to as an IC or chip, forms the backbone of the
electronics revolution. The typical integrated circuit comprises many transistors and other
components. As its name implies, the integrated circuit is a discrete and wholly functioning
circuit in its own right. ICs are the building blocks of larger circuits. By merely stringing them
together you can form just about any project you envision.

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