Independent Voltage Sources
There are sources in which
the voltage is independent of the current,
or the current is independent of the voltage.
These are termed independent sources
and are termed as active
elements.
An active element is an element that is
capable of furnishing an average power greater
than zero to some external devices, where
the average is taken over an infinite time
interval. Ideal sources and transistors
are active elements.
We use a circle enclosing the reference
polarity marks to represent independent
voltage sources. The value of the voltage
is indicated alongside the symbol. The voltage
can be constant or it can be a function
of time as shown in Figure 7.4.
Figure 7.4 Independent voltage sources
In Figure 7.4(a),
the voltage across the source is constant.
Therefore, we have a DC voltage source.
However, in Figure 7.4(b) we see an AC voltage
source having a sinusoidal variation with
time.
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