Nearly all piezos have two joints, but this type has three. They are known as self drive types, and they're meant to be used as part of the oscillator.

The piezo effect works both ways: in case you apply a voltage the piezo stretches, but also if it stretches them creates a voltage. This principle is needed to create a suggestions signal which drives your oscillator.

The advantage of the self drive is so it will automagically work with its resonance frequency, in which it produces the loudest appear.
Murata is active inside R&D of new vapor components, seeking infinite options with ceramic materials.
Especially, as a pioneer in the development of piezoelectric ceramics, Murata has met the needs belonging to the
 technical revolution in electronics with original products. Our ceramic resonators (CERALOCK®), ceramic filters
 (CERAFIL®), piezo sound components plus various ultrasonic transducers have contributed towards
 development of electronics. The "Piezoelectric sound components" introduced herein operate on an innovative principle working with natural oscillation of piezoelectric ceramics. These days, piezoelectric sound components are widely-used in many ways like home appliances, OA equipment, audio equipment telephones, and so on. And they are employed widely, for example, throughout alarms, speakers, telephone ring tones, receivers, transmitters,
beep appears to be, etc. This manual is made for customers to use piezoelectric sound components efficiently adequate no trouble. It is recommended which the manual be read while looking at the catalog
 MAS6240 is a piezo driver IC designed to create a multi-tone sound influenced by a charge pump know-how. It is a higher efficiency, easy to use and low cost IC perfect for power driven equipment or alternative low voltage applications where a high sound pressure is essential.

MAS6240 can drive outputs as much as 18 VP-P from solitary 3 V supply voltage. An input voltage amplification factor is usually selected as 1, Two or tree. The output can end up being single mode or differential style.

The inductorless design reasons significantly less disturbance to the surrounding circuits, making it the perfect choice Heart rate Monitors and various sensitive designs. Only four small value capacitors are needed besides a sound element. The use of small capacitors also creates this solution more reliable as compared to an inductive solution using a heavy coil that can brake loose when the equipment is dropped.

MAS6240 will also be used in other applications. For example as some sort of white LED driver. Together it can be linked to a piezo sounder inside a differential output configuration.
https://www.kailitech.net/Self-Drive-Piezo-Buzzer-pl3604953.html

Self Drive Piezo Buzzer
201911ld