Consider the Piezoelectric transformer, which is in close-loop configuration.
|The power is
controlled by a frequency shift. So, in close loop, we measure the output
voltage. The error signal is shifting the frequency.
Piezoelectric Transformer is, basically, resonant network, [>>] the
frequency shift will be translated into an Amplitude Modulation. So, for
designing the feedback loop there is a need to get a transfer function
between the frequency shift and the envelope change.