S-Band High Passive Gain Resonant Transformers Based on Aluminum Nitride FBAR Resonators

This paper investigates a passive voltage amplification in GHz range based on piezoelectric resonators with decent electromechanical coupling (kt²) and high quality factor (Q). The analytical model shows that keeping the transducer capacitance (C₀) sufficiently small is the key to attaining high passive voltage gain at GHz frequencies. To verify the proposed idea, two aluminum nitride (AlN) thin-film bulk acoustic wave resonators (FBAR) at 2.3 GHz with C₀ of only 133 fF (Type A, FoM = kt²·Q = 120) and 355 fF (Type B, FoM = 172) were implemented. As a result, high voltage gains of 21 dB (Type A) and 19 dB (Type B) have been demonstrated with reactive loadings of 120 fF and 140 fF, respectively, while achieving good input matching (S11 < -15dB). To the authors’ knowledge, this is the first MEMS-based resonant transformer demonstrated in S-band.