Charge amplifiers convert the charge output by a piezoelectric sensor into a proportional voltage, which is used as an input variable for analysis systems and can be digitized in an analog-to-digital (A/D) converter if necessary. A charge amplifier basically consists of an inverting voltage amplifier with a high open-loop gain and capacitive negative feedback. It relies on a metal oxide semiconductor field-effect transistor (MOSFET) or a junction field-effect transistor (JFET) at its input to achieve high insulation resistance and minimize leakage current. If the open-loop gain is sufficiently high, the cable and sensor capacitance can be neglected, leaving the output voltage dependent on just the charge at the charge amplifier input and the range capacitor

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