Electric circuit has many elements like resistor, capacitor, inductor, battery etc. Circuit analysis is the process by which voltage or current is measured across the element. In a complete circuit there are two types of elements found active elements and passive elements.
The active elements generate energy. Batteries, generators, operational amplifiers etc are active elements. The passive elements cannot generate energy, they drop energy. Resistor, capacitor, inductor etc are passive elements because they takes energy from circuit.
In a complete circuit voltage or current source are most active elements which deliver power in the circuit. Two type of source i) independent and ii) dependent source.
An ideal independent source is an active element that provides a specified voltage or current that is completely independent of other circuit elements.
An ideal voltage source is that element which supply voltage between two terminals to maintain current through the circuit. Generators, batteries are the ideal voltage sources in circuits. In figure two dc voltage sources is shown. Figure 1 (a) shows the symbol of time varying voltage source and figure 1(b) shows continuous dc voltage source.
An ideal independent current source is also an active element which supply a specified current to a circuit. Figure 2 shows the independent current source symbol where arrow sign indicates the direction of flowing current i.
An ideal dependent source is an active element in which the source quantity is controlled by another voltage or current.
Figure 3 shows the symbol of dependent voltage and current source it is diamond shaped. Dependent source of voltage or current is controlled by other element in the circuit. There are four types of possible dependent are
i) A current controlled voltage source (CCVS),
ii) A voltage controlled voltage source (VCVS),
iii) A current controlled current source (CCCS),
iv) A voltage controlled current source (VCCS).
In figure 4 a circuit diagram on the right hand side a current controlled voltage source is shown. Here the voltage is 5i depends on current i through element C. The value of dependent voltage source is in 5i Volt not in 5i Ampere. Because it is a voltage source. Voltage source has polarities (+ -) symbols. But current source contains arrow indicating the direction of current. An ideal dependent or independent voltage source produces any current requires to ensure that the terminal voltage is as stated. On the other hand an ideal current source produces the necessary voltage to ensure the stated current flow. In this way an ideal source could in theory supply an infinite amount of energy. One thing keep it mind that sources not only supply power to a circuit, they can absorb power from a circuit too.