Diodes exhibit capacitance that varies with respect to their DC bias. I know this is used in GHz RF applications.
Using opamps to create virtual reactive components (capacitors, or inductors) that would otherwise be impractical to build directly (too high of value) is an interesting trick.
Using opamps to create virtual reactive components (capacitors, or inductors) that would otherwise be impractical to build directly (too high of value) is an interesting trick.