A voltage source is a source of energy which establishes a potential difference across its terminals. Most of the sources encountered in everyday life are voltage sources, e.g. batteries, d.c. generators, alternators, etc. The current source is a source of energy that provides a current e.g., collector circuits of transistors. Voltage and current sources are called active elements because they provide electrical energy to a circuit.
Ideal voltage Source
An ideal voltage source is one which maintains a constant terminal voltage, no matter how much current is drawn from it. An ideal voltage source has zero internal resistance.
Practical Voltage Source
A practical voltage source has very low internal resistance, that causes its terminal voltage to decrease when load current is increased and vice-versa.
Ideal Current Source
An ideal current source is one which maintains will supply the same current to any resistance connected across its terminals. An ideal current source has infinite internal resistance.
Practical Current Source
A practical current source has very high internal resistance. Therefore, the load current will change as the value of load resistance changes.