Editing Inductance (section)

===Resonant transformer===
{{main|Resonant inductive coupling}}
When a capacitor is connected across one winding of a transformer, making the winding a [[tuned circuit]] (resonant circuit) it is called a single-tuned transformer. When a capacitor is connected across each winding, it is called a [[double tuned|double tuned transformer]]. These ''[[Transformer types#Resonant transformer|resonant transformers]]'' can store oscillating electrical energy similar to a [[resonant circuit]] and thus function as a [[bandpass filter]], allowing frequencies near their [[resonant frequency]] to pass from the primary to secondary winding, but blocking other frequencies. The amount of mutual inductance between the two windings, together with the [[Q factor]] of the circuit, determine the shape of the frequency response curve. The advantage of the double tuned transformer is that it can have a wider bandwidth than a simple tuned circuit. The coupling of double-tuned circuits is described as loose-, critical-, or over-coupled depending on the value of the [[Coupling coefficient (inductors)|coupling coefficient]] {{nowrap|<math>k</math>.}} When two tuned circuits are loosely coupled through mutual inductance, the bandwidth is narrow. As the amount of mutual inductance increases, the bandwidth continues to grow. When the mutual inductance is increased beyond the critical coupling, the peak in the frequency response curve splits into two peaks, and as the coupling is increased the two peaks move further apart. This is known as overcoupling.

Stongly-coupled self-resonant coils can be used for [[wireless power transfer]] between devices in the mid range distances (up to two metres).<ref name="Kurs">{{cite journal |last1=Kurs |first1=A. |last2=Karalis |first2=A. |last3=Moffatt |first3=R. |last4=Joannopoulos |first4=J. D. |last5=Fisher |first5=P. |last6=Soljacic |first6=M. |title=Wireless Power Transfer via Strongly Coupled Magnetic Resonances |journal=Science |date=6 July 2007 |volume=317 |issue=5834 |pages=83–86 |doi=10.1126/science.1143254 |pmid=17556549 |bibcode=2007Sci...317...83K |citeseerx=10.1.1.418.9645 |s2cid=17105396 }}</ref> Strong coupling is required for a high percentage of power transferred, which results in peak splitting of the frequency response.<ref>{{cite journal|doi=10.1109/TIE.2010.2046002|title=Analysis, Experimental Results, and Range Adaptation of Magnetically Coupled Resonators for Wireless Power Transfer |year=2011 |last1=Sample |first1=Alanson P. |last2=Meyer |first2=D. A. |last3=Smith |first3=J. R. |journal=IEEE Transactions on Industrial Electronics |volume=58 |issue=2 |pages=544–554 |s2cid=14721 }}</ref><ref>{{cite book|doi=10.1109/MEMS51670.2022.9699458|chapter=Magnetically Coupled Microelectromechanical Resonators for Low-Frequency Wireless Power Transfer |title=2022 IEEE 35th International Conference on Micro Electro Mechanical Systems Conference (MEMS) |year=2022 |last1=Rendon-Hernandez |first1=Adrian A. |last2=Halim |first2=Miah A. |last3=Smith |first3=Spencer E. |last4=Arnold |first4=David P. |pages=648–651 |isbn=978-1-6654-0911-7 |s2cid=246753151 }}</ref>