Versatile preamplifiers for use with Piezoelectric sensors. The need for a Piezo Film Lab Amp explained:

Charge Mode: In a charge mode preamplifier, the input appears to be a short circuit. The potential between the two input terminals is maintained at exactly zero volts. All of the charge generated by the piezo sensor flows into the circuit, and is transferred to a feedback capacitor. The rate of charge decay is then governed by this selected capacitor, and not by the capacitance of the piezo source. Since the input is "shorted", stray capacitance has no effect.  Use charge mode when:

• Piezo sensor has long cable attached (or varying cable length)
• Only a portion of the active sensor area is impacted or excited
• "Fundamental" measurement of charge output preferred
• Minimize triboelectric cable noise

Voltage Mode: In a voltage mode preamplifier, the input appears to be an infinite impedance. The potential between the two input terminals is maintained at the "open circuit" potential of the source network (including all stray capacitance). None of the charge generated by the piezo sensor flows into the circuit. A bleed resistor is required on the input (otherwise circuit would be unstable or saturate), and the low frequency response is governed by the total capacitance connected to the input, and this bleed resistance.  Use voltage mode when:

• Lab Amp will later be replaced by simple buffer stage
• "End of cable" output voltage is preferred