A P P E N D I X B
ELECTRICAL
CIRCUITS REVIEW
by Bruce M. Fleischer
This appendix is recommended reading if you want a review of electronics’ basic
ideas. Why, you may ask, does a digital circuit designer need to know this analog
“stuff”? You need it to understand the abilities and shortcomings of components
available in the real world, which must be used in building circuits that will work
in the real world.
A practical problem that motivates this appendix is the calculation of the
rise and fall times of a logic circuit output driving one or more inputs of other
logic circuits (see Section 3.6 of the text). That problem is really one of analog,
not digital circuits. Therefore, much of this appendix consists of building up
enough ideas from analog electronics to make a simple model for rise and fall
times.
B.1 FUNDAMENTALS
B.1.1 Charge
In the Bohr theory of the atom (named after Niels Bohr, 1885–1962), electrons
electron
orbit a nucleus containing neutrons and protons. Attraction between the opposite
neutron
proton
charges of electrons and protons keeps atoms together. Particles with the same
charge repel each other. You may wonder what attraction holds the nucleus
together, with all those protons. The answer is that the physicists have it all
under control, and be glad this isn’t an appendix on particle physics.
B–1
B–2
ELECTRICAL CIRCUITS REVIEW
APP. B
The nearly equal numbers of electrons and protons in most objects, such
as a piece of fur, cancel out, so electrons in a neighboring object, such as an
amber rod, usually don’t feel any overall attraction for the fur. In some atoms,
however, the electrons are not as tightly held as in others, and there are situations
in which some electrons come loose.
For example, if we rub the amber rod on the fur, some of the electrons
from the fur end up on the amber. The fur is missing some electrons, while the
amber has extras. You can see the attraction of these charges for each other; the
hairs in the fur are pulled towards the rod. If the two are brou