The name “passive circuit” can be a bit misleading. It refers to a circuit containing only passive components such as resistors, capacitor, and inductors. For such a circuit to do anything meaningful, it still need a source of energy. It might not be AC mains or a battery, but it must be there.
For example, the energy source for dynamic microphones is sound waves. The microphone generates electric current through electromagnetic induction powered by sound waves moving a diaphragm connected to a magnet inside a coil.
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