Driving piezoelectric crystals, low to high or matched impedance?

Question

When designing an interface between a fixed frequency RF generator and a piezoelectric crystal, should the aim be to match the load impedance to the source impedance (for maximum power transfer). Or should there be a low to high impedance (maximum voltage transfer).

It seems intuitive to me that a matched impedance should be required, as ultimately I am aiming for maximum acoustic power. However when researching the subject, I only seem to read about electrical energy mentioned in terms of voltage, not power. Which leads me to think that I should be aiming for a low to High impedance.

What should I be aiming for, maximum power transfer to the transducer or maximum voltage transfer to the transducer?

Answer 1

It depends on how the amp is connected to the load...

If the cable is short enough relative to wavelength not to be a transmission line, and its inductance is small enough, then you will get maximum power into your load by using a low source impedance.

However if your cable is long and becomes a transmission line at your frequency, then you have to match impedances, or else power will be reflected at the load.

So, both answers are right, and the deciding factor is the cable length.

Answer 2

It depends how hard ou want to drive it, and over what bandwidth.

Generally, piezoelectric crystals are capacitive. Generally, the current needed to charge that capacitance at the desired frequency of operation is higher than we'd like.

If we only need a single frequency of operation, and need a lot of power, say to drive an ultrasonic cleaner, then we tend to resonate the transducer with an inductor, and tune it to the operating frequency. This allows the whole circuit to present a resistive impedance.

If we want to drive a range of frequencies, then we need to design the tuning components as a bandpass filter.

Answer 3

Matching impedances for maximum power transfer only applies for a fixed source impedance and a variable load impedance. If the load impedance is fixed, then the maximum power will be transferred with a zero source impedance so that the full source voltage will be applied to the load. In your case, with a fixed load impedance, your crystal, the maximum power will be transferred with the minimum source impedance. However, if other considerations exist, such as the need to match to a long transmission line, then this does not apply.