What type of circuit is a crystal-controlled oscillator?

A crystal-controlled oscillator is classified as an electronic circuit that contains a piezoelectric crystal. This type of oscillator relies on the specific resonant frequency of the crystal, which is made from materials that exhibit piezoelectric properties. When voltage is applied to this crystal, it vibrates at a specific frequency, thus generating a stable and precise signal output.

The piezoelectric crystal's ability to resonate at certain frequencies allows the oscillator to produce a consistent output signal with minimal frequency drift, making it ideal for applications in communication systems, clocks, and various electronic devices. This stability is crucial for timing and synchronizing operations in many electronic systems.

Other circuit types listed in the choices, such as those involving diodes, AC to DC conversion, or transformers, do not fundamentally focus on the piezoelectric properties or the crystalline structure needed for the function of a crystal-controlled oscillator. Thus, understanding the properties and role of the piezoelectric crystal is key to recognizing why the correct classification is this option.