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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