All piezo devices operate within a range of frequencies known to the FCC and the broadcasting world as the RF band.   The FCC breaks the RF band down into several frequency categories, the most commonly experienced when working with piezos being the Very Low Frequency (VLF) band (3-30 kHz), the Low Frequency (LF) band (30-300 kHz), the Medium Frequency (MF) band (300-3000 kHz) and the High Frequency (HF) band (3-30 MHz).  Generally speaking when referring to piezo devices, we can simplify these 4 frequency categories down to 2 and consider a piezo device either a "Low Frequency" or a "High Frequency" device.  A low frequency piezo device is one which would typically operate from direct current (DC) to approximately 100 kHz, while a high frequency piezo device would be one which operates from approximately 100 kHz to 20 MHz.

 

A rough breakdown of some typical devices and the frequencies where the operate would be (Figure 1):

Powering a piezo device is not complicated.  Shown below in a simplified block diagram (figure 2) is the flow required to be successful in driving a piezo device.

Figure 2 - A simple diagram for powering piezo devices

Input Signal:

The input signal is simple.  Create a signal at the frequency for which the piezo device was designed to function.  This can be done with a frequency generator, waveform generator, oscillator, or even a computer with a USB adapter.  Once you have a signal, it must be amplified to a power level adequate for driving the piezo device.

 

RF Power Amplifier:

Amplifiers can be confusing, but hopefully we can simplify things.  By focusing our discussion on powering a piezo device (in a displacement or transmitting mode) in an engineering application (to evaluate the design of a transducer for example), we can target several types of amplifiers to further simplify the discussion.  Once the design is sound, custom power circuits can be designed to maximize efficiency, minimize size, and generally suit the specific application. 

 

Power amplifiers are broken down into several general "categories" and are denoted by the letters A, B, AB, C, D etc.  For the purposes of this discussion, we will focus on A, B, AB, and D as they are the most commonly used styles of power amplifiers when engineering piezo devices.

 

(A) Power Amplifiers:

In an (A) type power amplifier, your signal input is directly and fully amplified (all 360 degrees of the input cycle) and delivered as a higher power output with good signal quality and little distortion.  This sounds fantastic, but it comes at a cost.  Efficiencies of (A) type power amplifiers rarely approach 50%, but despite low efficiency, the high signal quality and direct amplification of the input signal make the (A) type amplifier a great engineering amplifier for working with and designing piezo devices.

 

(B) Power Amplifiers:

Essentially ½ of an (A) type amplifier, the (B) type amplifier increases efficiency to 75% by amplifying only ½ of the input signal (180 out of 360 degrees of the input cycle).  These are specialty amplifiers that are rarely used alone, but commonly used as a component in another amplifier such as the AB amplifier which will be discussed in the next section.

 

(AB) Power Amplifiers:

Think of an (AB) amplifier as 2-(B) type amplifiers working together in a push pull arrangement to achieve full signal amplification at higher efficiencies than the (A) type amplifier.  (AB) amplifier efficiencies can approach 70%.  One (B) type amplifier handles ½ the input signal, and an additional (B) type amplifier handles the remaining ½ of the signal input.  One drawback of the (AB) amplifier is that a certain degree of distortion at the crossover region of the input signal is to be expected, and is not always desirable when powering piezos.

 

(D) Power Amplifiers:

Aka the Switch Mode Amplifier, the (D) type amplifier does not simply amplify the input signal.  In a very basic sense, the switch mode amplifier switches ON and OFF with the input signal.  This switching typically occurs at a frequency a factor of 10 higher than the input signal frequency.  This essentially converts the input signal to a digital one, amplifies it, than converts it back to an analog output.  Despite not directly amplifying the input signal to the output signal, the (D) type amplifiers have relatively low power requirements and are capable of operating at 90% or greater efficiencies.  These are the most complicated and most expensive amplifiers we have covered in our discussion.

 

Impedance Matching Circuit:

Impedance matching circuits may or may not be necessary, depending on the frequency of the device you are driving.  Low frequency piezo devices (DC to 100kHz) are often used in non-resonant applications.  Additionally, power amplifiers in these ranges are typically not load dependent and are designed for impedance mis-match.  The impedance of the piezo device (load) in this case does not dramatically affect the amplifiers performance. 

 

Driving high frequency piezo devices (100 kHz - 20 MHz), on the other hand, requires precise matching of the device impedance to the amplifier output.  This can be achieved to an acceptable degree of accuracy using an adjustable impedance matching device or by designing a custom matching circuit.  APC offers impedance matching boxes available with an input impedance of 50 ohms and an adjustable output impedance consisting of 4 possible impedance values up or down to cover a wide variety of piezo devices.  APC also offers custom matching circuits which can be supplied for a specific application upon request.

In a slightly more complex arrangement, Figure 3 shows a block diagram of a setup for measuring acoustic output by powering a piezo transducer.  In this diagram, some additional equipment including measuring devices such as an oscilloscope, power meter, and special probes can be seen.  Putting this diagram into practice results in Figure 4, a benchtop setup for powering a piezo transducer and measuring its acoustic output power.

Figure 3 - Slightly more complex diagram for powering piezo

transducer and measuring its output power

Figure 4 - In Practice - Benchtop setup matching figure 3 diagram

for powering piezo and measuring acoustic output power

As you can see, by breaking this down into 3 major categories (signal generation, amplification, impedance matching) the task of powering piezos for engineering applications becomes far less complicated.  

APC is proud to offer a line of SVR amplifiers for powering actuator devices (DC-1 kHz), RF amplifiers (seen in figure 4) for higher frequency devices (20 kHz - 1MHz @ 100W) and impedance matching boxes (also in figure 4) to take the mystery out of powering piezo devices.   Custom power amplifiers are available through APC by request, and APC is currently offering new old stock of several  High Power LE Amplifier Systems, but stock is limited so order soon!!!  Please visit www.americanpiezo.com for more information or contact your APC Technical Sales Representative with any questions, comments or requests.

 

Next month we will be taking an in depth look at the devices we are powering.  After Manufacturing - "Anatomy of a Transducer" will dive into the components which make up a transducer and the role they play in the final device.