Piezoelectric actuators offer many benefits over traditional solenoid counterparts. Piezoelectric actuators are more precise, power efficient, and smaller in general when compared to solenoid actuators. One challenge with piezoelectric actuators is that the piezoelectric actuators operate over a very wide voltage range. Driving voltages of more than 60V are not uncommon, and can easily reach hundreds of volts. The OPAx596 operate with a supply voltage of up to 85V.

In this design example, the OPAx596 are used to provide a 100V_PP signal to control a high-voltage piezoelectric actuator (see also Figure 7-1). The piezoelectric actuator can be modeled as a large capacitor when operated at less than the resonant frequency. The piezoelectric actuator is treated as a floating load driven by two op amps of the OPAx596. The outputs of the op amps are set to be 180° out-of-phase to essentially double the voltage seen by the actuator load. The signal voltage of the digital-to-analog converter is applied a −10V/V gain by the OPAx596. A simple voltage divider provides a dc reference to level shift the output to get a unipolar driving voltage. Figure 7-2 shows the output voltage of both amplifiers and the voltage seen by the piezoelectric load.

The large capacitive load seen by the amplifiers can lead to instability and proper compensation is required. A straight forward method to improve phase margin and stability is to add isolation resistors and compensation capacitors in the feedback. A small 10Ω R_ISO at the output of each of the OPAx596 and a 1nF compensation capacitor is effective. Keep the isolation resistors as small as possible to minimize the voltage drop across them. Choose the compensation capacitors according to the frequency of operation. For this example, 1nF capacitors leave enough bandwidth to accommodate a 100Hz signal. Use simulation tools, such as PSPICE or TINA-TI, to confirm stability.

Understanding the limitations of this circuit are important. As with any capacitive load, the impedance can significantly decrease at higher frequencies. This behavior greatly increases the current output capability requirements of the driver amplifier at high frequencies. If high frequency operation is required, consider other amplifiers with higher current drive capability. At 100Hz, the OPAx596 is capable of supplying the necessary current. Figure 7-3 shows the current output of the OPAx596 in this example.