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Design Techniques to Increase a Piezo Transducer Buzzer Audio Output

By Ryan Smoot, Technical Support Engineer, CUI Devices

Used across a range of applications and industries as a means for audible identification or alert, piezo transducer buzzers are able to create variable tones and sounds depending on the specific need of an application. The amplitude of sound produced by a piezo transducer buzzer is contingent upon both the specific buzzer selected and the signal used to drive the buzzer. Because transducer buzzers require an external driving circuit to produce a tone or sound, there are several methods to impact the audio output of a piezo buzzer based on the design of the external driver circuit. While simple in practice, this article aims to provide a primer on a piezo transducer’s working principles as well as the advantages and limitations of common design techniques for increasing a transducer’s audio output.

Piezo transducer working principles

CUI Devices’ technical paper on buzzer basics provides an in-depth overview on piezoelectric transducers, but here is a quick refresher on the technology. A piezoelectric device is constructed of a material that physically deforms when a voltage is applied across the device, where the amount of deformation and the resultant noise volume caused by the deformation are related to the voltage applied across the piezo material. As mentioned earlier, a transducer buzzer requires an external excitation signal to operate. Indicator buzzers on the other hand only require a supply voltage to operate due to an internal oscillator. This can make indicators easier to design in, but also limits the types of tones and sounds produced compared to a transducer.

Simple driver circuit

Shown in the circuit diagram below (Figure 1), is one of the simpler driver circuits for a piezo transducer buzzer, which is composed of an electronic switch, such as an FET or BJT, and a reset resistor. As this circuit requires only a few, inexpensive parts, it can be a popular choice for more basic designs. But, while simple, this design does have its drawbacks in that the reset resistor dissipates power and the voltage applied to the buzzer is limited to the supply voltage (+V). Please note that the buzzer and circuit will function the same regardless of whether the one buzzer terminal is connected to the +V supply (as shown in Figure 1) or to ground.

Diagram of driving circuit composed of an electronic switch and reset resistorFigure 1: Driving circuit composed of an electronic switch and reset resistor. (Image source: CUI Devices)

Driver circuit with buffers

An engineer can reduce the power loss of the reset resistor from the previous driver circuit with the addition of two buffer transistors (Figure 2). These two buffer transistors allow for the use of a higher impedance reset resistor at the cost of a reduced voltage applied to the buzzer of approximately two diode drops, or about 1.2 V. Again, similar to the Figure 1 circuit, this buzzer and circuit with the added buffers will function the same regardless of whether the one buzzer terminal is connected to the +V supply or to ground.

Diagram of driving circuit with two added buffersFigure 2: Driving circuit with two added buffers. (Image source: CUI Devices)

To address the issue of reduced voltage an engineer can simply reverse the positions of the BJT buffers used above. This circuit can also be constructed with FETs instead of BJTs as the buffer components. Both buffer configurations are outlined in Figure 3.

Diagram of position of BJT buffers reversed (left) or FET buffers in place of BJTs (right)Figure 3: Position of BJT buffers reversed (left) or FET buffers in place of BJTs (right). (Image source: CUI Devices)

Half- and full-bridge drivers

While the changes to the buffer configurations mentioned above (Figure 3) are an option, they will make the driver circuits for the buffers more complex, which may not be desired when designing with discrete components. This form of driver with push-pull buffers is commonly referred to as a “half-bridge” driver. A buzzer can be connected between the outputs of two half-bridge drivers and when these two half-bridge drivers are driven out of phase, they are known as a “full-bridge” driver. Both half-bridge and full-bridge drivers are often used to drive electric motors and are available as inexpensive integrated circuits. Full-bridge drivers also offer the benefit of delivering two times the voltage to the buzzer as a basic driver or half-bridge driver, which results in a louder sound output using the same supply voltage as other solutions.

Diagram of full-bridge driver circuitFigure 4: Full-bridge driver circuit (Image source: CUI Devices)

Resonant driver circuit

Due to the parasitic capacitance present in transducer buzzers, engineers have an additional option for driving a piezo transducer by utilizing a discrete inductor to form a resonant circuit. Resonant circuits store and transfer energy alternately between two elements; with the two elements in this application being the parasitic capacitor and the inductor. Figure 5 shows one such implementation of a resonant driver circuit for a piezo transducer buzzer.

Resonant driver circuits offer several benefits, including simple construction and the potential for high electrical efficiency. The voltage developed across the piezo buzzer can also be much larger than the supply voltage. However, the resonant driver circuit can be hampered by the fact that it is reliant upon a piezo transducer’s parasitic capacitance, which during the manufacturing process is not always well characterized or controlled. Resonant piezo transducer driver circuits also only perform well at one specific frequency, making them less suitable for applications requiring multiple frequency tones. In addition, the selected operating frequency impacts the inductor, which can be physically large and heavy compared to other circuit components. Modeling the operation of the resonant circuit can also be difficult, meaning the circuit may need to be finalized in the lab rather than at the design computer.

Diagram of resonant driver circuitFigure 5: Example of a resonant driver circuit (Image source: CUI Devices)

Conclusion

An engineer has many options when it comes to designing a driver circuit for a piezoelectric transducer buzzer. From using simple discrete components to more complex circuit designs, each driver comes with its own set of trade-offs for reaching an application’s desired sound output. Once the key performance parameters are determined, CUI Devices makes the selection process easy with a range of piezo and magnetic buzzers readily available to meet a design’s requirements.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Ryan Smoot, Technical Support Engineer, CUI Devices

With an extensive knowledge of CUI Devices' products, Ryan Smoot provides customers with a wide range of technical and application support capabilities in the field. His management of CUI Devices' robust CAD model library further offers engineers with an invaluable resource for streamlining their product designs.