Signal source frequency = PZ1, resonant frequency = 40KHz.This is demonstrated in Figure 4, where the waveforms are derived using the following circuit parameters: In fact, if it is roughly 2× the acoustic resonance, the peak voltage across the transducer can be greatly increased. The electrical resonance frequency need not be the same as that of the acoustic resonance. Electrical resonance of the inductance and transducer capacitance.Acoustic resonance of the transducer, mechanical and cavity resonances apply.To achieve any further improvements, we need to consider that there are actually two resonances at work in this small system: Although the forward voltage of the diode does reduce the applied power supply voltage, the increased resonance voltage more than makes up for this small loss.įigure 3 Using a diode can eliminate the circuit’s negative swing. That’s because the transistor collector-base junction is forward biased on the negative swing of the parallel resonant circuit formed by the inductor and transducer capacitance, which clamps the voltage swing, limiting acoustic output.Īdding a diode decouples the C-E junction (or if a FET is used, the body diode junction) from this negative swing, providing a much larger voltage swing across the transducer, increasing the acoustic output ( Figure 3). At best, the peak-to-peak voltage across the transducer may reach 40Vppk, while 20Vppk is more typical with a 5V power supply. This approach can provide more acoustic output than the parallel resistor approach, but there is still a lot of room for improvement. Often the inductance value is selected to electrically resonate with the capacitance of the transducer (buzzer) at the acoustic resonance of the transducer. Low resistor values decrease electrical efficiency while damping the mechanical (acoustical) resonance of the transducer, which of course reduces the acoustic efficiency.Ī common enhancement to this circuit replaces R2 with an inductor as shown in Figure 2.įigure 2 Substituting an inductor for R2 improves the piezo driver’s output and efficiency. The RC time constant should be short relative to the period of the resonant frequency of the transducer. The resistor R2 serves to discharge the capacitance of the transducer. The voltage across the transducer cannot be greater than the supply voltage, which places an upper limit on the acoustic output. The simplest piezo drive circuit consists of a transducer and a switching transistor ( Figure 1). Before we look at the new approach, let’s look at some of the most commonly-used piezo-acoustic designs and their drawbacks. In contrast, this Design Idea shows how the acoustic output of a piezoelectric transducer can be increased while minimizing the parts count and cost. Most of them involve rather complicated circuitry that drives up the total solution cost such as boosting the low-voltage logic power supply to a higher voltage or using an H-bridge topology. There have been many different ideas presented for increasing the acoustic output of a piezo buzzer or ultrasonic transducer.
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