The elements that supply energy to the circuit is called active element.
Examples of active elements include voltage and current sources, generators, and
electronic devices that require power supplies.
A transistor is an active circuit element,
meaning that it can amplify power of a signal. On the other hand, transformer is not an
active element because it does not amplify the power level and power remains same both in primary and secondary sides.
Transformer is an example of passive element.