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256 ⏐⏐⏐ METHODS OF ANALYSIS AND SELECTED TOPICS (dc)
gate its characteristics in greater detail so that we can properly determine
its effect on the networks to be examined in this chapter.
The current source is often referred to as the dual of the voltage
source. A battery supplies a fixed voltage, and the source current can
vary; but the current source supplies a fixed current to the branch in
which it is located, while its terminal voltage may vary as determined
by the network to which it is applied. Note from the above that duality
simply implies an interchange of current and voltage to distinguish the
characteristics of one source from the other.
The interest in the current source is due primarily to semiconductor
devices such as the transistor. In the basic electronics courses, you will
find that the transistor is a current-controlled device. In the physical
model (equivalent circuit) of a transistor used in the analysis of transistor
networks, there appears a current source as indicated in Fig. 8.1. The symbol
for a current source appears in Fig. 8.1(a). The direction of the arrow
within the circle indicates the direction in which current is being supplied.
B
C
E
=
B I b
Current source
(a) Transistor symbol (b) Transistor equivalent circuit
FIG. 8.1
Current source within the transistor equivalent circuit.
For further comparison, the terminal characteristics of an ideal dc
voltage and current source are presented in Fig. 8.2, ideal implying perfect
sources, or no internal losses sensitive to the demand from the
applied load. Note that for the voltage source, the terminal voltage is
fixed at E volts independent of the direction of the current I. The direction
and magnitude of I will be determined by the network to which the
supply is connected.
E
I
I
E
Voltage
0 I
(a)
FIG. 8.2
Comparing the characteristics of an ideal voltage and current source.
βr e
E
(–) +
I Vs (+) –
βI b
C
I
0
(b)
Current
The characteristics of the ideal current source, shown in Fig. 8.2(b),
reveal that the magnitude of the supply current is independent of the
polarity of the voltage across the source. The polarity and magnitude of
the source voltage V s will be determined by the network to which the
source is connected.
For all one-voltage-source networks the current will have the direction
indicated to the right of the battery in Fig. 8.2(a). For all single-
N A
V s