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ACTIVE CIRCUIT ELEMENTS (ENERGY SOURCES)

An active two-terminal element that supplies energy to a circuit is a source of energy. An ideal voltage source is a circuit element that maintains a prescribed voltage across the terminals regardless of the current flowing in those terminals. Similarly, an ideal current source is a circuit element that maintains a prescribed current through its terminals regardless of the voltage across those terminals. These circuit elements do not exist as practical devices, they are only idealized models of actual voltage and current sources.
Ideal voltage and current sources can be further described as either independent sources or dependent sources. An independent source establishes a voltage or current in a circuit without relying on voltages or currents elsewhere in the circuit. The value of the voltage or current supplied is specified by the value of the independent source alone.

In contrast, a dependent source establishes a voltage or current whose value depends on the value of the voltage or current elsewhere in the circuit. We cannot specify the value of a dependent source, unless you know the value of the voltage or current on which it depends.

The circuit symbols for ideal independent sources are shown in Figure (a) and (b).

Note that a circle is used to represent an independent source. The circuit symbols for dependent sources are shown in Figure (c), (d), (e) and (f). A diamond symbol is used to represent a dependent source.
Figure: (a) An ideal independent voltage source (b) An ideal independent current source (c) Voltage controlled voltage source (d) Current controlled voltage source (e) Voltage controlled current source (f) Current controlled current source

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