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ElliotTheUnread
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A perfect voltage source is a two-terminal black box that maintains a fixed voltage
drop across its terminals, regardless of load resistance. For instance, this means
that it must supply a current I = V/R when a resistance R is attached to its
terminals. A real voltage source can supply only a finite maximum current, and in
addition it generally behaves like a perfect voltage source with a small resistance in
series. Obviously, the smaller this series resistance, the better. For example,
a standard 9 volt alkaline battery behaves like a perfect 9 volt voltage source in
series with a 3 ohm resistor and can provide a maximum current (when shorted) of 3 amps
(which, however, will kill the battery in a few minutes). A voltage source "likes" an
open-circuit load and "hates" a short-circuit load, for obvious reasons. (The terms
"open-circuit" and "short-circuit" mean the obvious: An open circuit has nothing connected
to it, whereas a short circuit is a piece of wire bridging the output.) The symbols used to
indicate a voltage source are shown in Figure 1.7.
A perfect current source is a two terminal black box that maintains a constant
current through the external circuit, regardless of load resistance or applied
voltage. In order to do this it must be capable of supplying any necessary
voltage across its terminals. Real current sources (a much-neglected subject in
most textbooks) have a limit to the voltage they can provide (called the output
voltage compliance, or just compliance), and in addition they do not provide absolutely
constant output current. A current source "likes" a short-circuit load and "hates" an
open-circuit load. The symbols used to indicate a current source are shown in Figure 1.8.
A battery is a real-life approximation of a voltage source (there is no analog for a current
source). A standard D-size flashlight cell, for instance, has a terminal voltage of 1.5
volts, an equivalent series resistance of about 1/4 ohm, and total energy capacity of about
10,000 watt-seconds (its characteristics gradually deteriorate with use; at the end of its
life, the voltage may be about 1.0 volt, with an internal series resistance of several ohms).
It is easy to construct voltage sources with far better characteristics, as you will learn
when we come to the subject of feedback.