CHAPTER 16 ■ TRANSISTOR SWITCHES
Focusing on the 2907A Transistor
The 2907A transistor (see Figure 16-1) is more formally referred to as the 2N2907A. It is a bipolar PNP
general-purpose amplifier.
- Bipolar indicates the type of semiconductor technology. Bipolar semiconductors
are popular because they’re fast, static-electricity resistant, and can deliver plenty
of current. However, bipolar semiconductors use more energy (and therefore emit
more heat) than field-effect semiconductors. - PNP indicates that the transistor turns on with negative power. Just the opposite,
NPN transistors turn on with positive power. (Each letter indicates a layer of doped
silicon.) - General Purpose indicates that the transistor operates with characteristics similar to
most transistors. Compare that with high-speed, low-noise, or power transistors that
have special abilities tailored for specific duties. - Amplifier indicates the transistor can magnify a signal. Some other transistors are
designed more for switching, buffering, or acting as adaptors from one voltage to
another.
Like all bipolar transistors, the 2907A has three leads labeled emitter, base, and collector (not always in
that order). Transistor experts hook them up in bizarre ways to achieve funky abilities. However, this book
focuses on the more mundane, yet popular, arrangements. For the line-following robot, the 2907A transistor
will be acting as a switch controlled by the comparator’s output.
Figure 16-1. Fairchild 2907A transistor in a TO-92 package