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Driving a piezo - how?

Paul Lauria
Prodigy 10 points

Hi guys,

 

I need a way to drop about 5V peak-to-peak sine wave at ~50MHz  across a piezo actuator with a capacitance of 37 nF.

Can someone help me with the calculations here, and/or suggest an op amp...? Seems to me I'd need an op amp with output current capabilities of >45A  using V=IZ, with Z=1/2Pi f C. I don't think that's right - or at least I hope not! Any suggestions? /physics major woefully underknowledged about op amp design

  • 0
    Genius 5430 points

    Paul;

    You are going to need an RF power amplifier, not an op amp, to drive that load.

  • 0
    TI__Guru* 93105 points

    Hi Paul,

    Sorry I am slow to get back to you, but I needed to make sure I fully studied your inquiry before answering. Neil is correct - the application requires an RF amplifier rather than a closed-loop operational amplifier circuit. Here is why:

    Wide-band, high-speed operational amplifiers are not well suited for driving large capacitive loads such as 37nF. Your application requires an amplifier with a slew rate of about 800V/us and a peak current approaching a quarter ampere. There are operational amplifiers that can deliver that level of performance, but not when driving a 37nF load capacitance. First off, there is the possibility of instability due to the added pole in the feedback loop created by the operational output resistance and the load capacitor. Sometimes however, with such large capacitances, the circuit is unexpectedly stable. This comes about because the load capacitance in conjunction output impedance recompensates the operational amplifier. The problem with this outcome is it drastically reduces the bandwidth of the amplifier and its slew rate rendering it useless for the intended application.

    You may have to do some research into RF power amplifiers that are capable of driving piezoelectric transducers. Many RF power amplifier products are designed to deliver output power into a specific load impedance such as 50 Ohms. These same amplifiers may behave quite differently with a highly reactive load.

    Regards, Thomas

    PA - Linear Applications Engineering