My setup is this - OPA548 opamp - 1:20 transformer - piezo at 100KHz
I'm getting slew rate distortion when i output over 30Vpp from the opamp, driving the piezo at over 600Vpp. This is to be expected from the datasheet. The circuit itself is tried and tested over several months and is fully functional.
My question is this - if i add an inductor in series with the piezo thus reducing/cancelling the impedance of the piezo will I be able to drive the volts higher without slew rate distortion?
My thought is that theoretically I should be able to drive the piezo with more volts - but I tried this and it didn't seem to make any difference.
Regards,
Szzuk.
Hi,
The slew rate comes into play when the output voltage goes over a certain level at a certain frequency. In other words, at a certain frequency the maximum output voltage without distortion is limited by the slew rate of the op amp.
The maximum gain is limited by the gain bandwidth product, which means you might need a certain minimum level input voltage so you can use a low enough gain to satisfy the GBP.
For this op amp, 30v is probably too high of a voltage at 100kHz. It's a shame because at 50kHz it might work, but that's half what you want.
So to use this at 100kHz you'll have to reduce the output to 15v peak and use a higher ratio transformer.
Alternately you could use two of these op amps in a bridge configuration driving the second one 180 degrees out of phase with the first one, and connecting the transformer across the two outputs rather than one output and ground. The output of each one could then be only 15v but you'll get the full 30v across the two outputs. This should work at 100kHz.
Of course the gain bandwidth product has to be satisfied too, so max gain would be 10 with a 1.5v input, but better would be a gain of 5 with a 3v input for each op amp.
Please note in the above discussion the voltages mentioned are all in units of peak volts, not rms volts. This means not 15v rms but 15v peak. You may be able to stretch it to 15v rms but that's something you'll have to try.