US Patent 7141910 Transconductance circuit for piezoelectric transducer

A transconductance monitoring and amplifying circuit for a piezoelectric transducer that may be used in, e.g., a motion detector system includes a FET, with the transducer (and, hence, the signal voltage reference) floating between the gate and source of the FET, as opposed to being connected to the common ground of the circuit. This permits the development of a larger detector signal and concomitantly the use of a relatively inexpensive FET instead of a relatively more expensive high impedance operational amplifier as must be used in conventional transconductance circuits. The FET and transducer can be held in a single four-pin package.