The two figures above show the same piezo transducer driven over its resonance frequency with two different driving conditions, *constant voltage* and *constant current*.

The left figure shows a constant voltage drive - notice the "drooping over" of the admittance with increasing amplitude - this is a nonlinear phenomenon as power input strongly increases at resonance with constant voltage drive.

The figure on the right shows a constant current excitation scheme - notice that the response is symmetrical. At resonance, current very closely equates to input power, so by controlling current to be constant, we eliminate nonlinearity and thus get a symmetrical response. Yup, we can calculate Q-factor using traditional bandwidth calculations now. Phew!