Crystals are not applicable to which of the following?

Quartz crystals exhibit the piezoelectric effect, making them highly useful in various electronic applications. They are commonly used to create highly stable oscillators, in lattice filters to achieve very sharp band-pass characteristics, and in crystal microphones where sound pressure deforms the crystal to produce a voltage. Active filters, on the other hand, are constructed using active components like operational amplifiers (op-amps) combined with basic passive components such as resistors and capacitors. Because active filters rely on these amplifying components to shape the frequency response (typically at audio frequencies), they do not utilize quartz crystals in their design.