Charge amplifier
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A charge amplifier is an electronic amplifier which converts an input charge (stored on a capacitor, or significantly capacitive transducer) to a voltage output. A common application is for use with a piezoelectric sensor, where they convert the charge output from the transducer into a voltage. Charge amplifiers are often found in instrumentation, and in the readout circuitry of CCD imagers and flat-panel X-ray detector arrays. In read-out circuits the objective is usually to measure the very small charge stored within an in-pixel capacitor, despite the capacitance of the circuit-track to the readout circuit being a couple of orders of magnitude greater than the in-pixel capacitor.
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[edit] Explanation
A piezo electric transducer has very high DC output impedance and appears as a voltage source in series with a capacitor. A charge amplifier therefore needs to have a very high input impedance at DC.
[edit] Design
Charge amplifiers are usually constructed using op amps with a feedback capacitor. They thus act in a similar manner to an integrator. Since the transducer acts in a similar manner to a differentiator, the two transfer functions cancel and the output voltage is proportional to the charge produced by the transducer. Stray capacitance at the input to the amplifier is not detrimental to operation because this capacitance is always at a virtual ground.
[edit] Applications
- Accelerometer signal conditioning
- Guitar pickup amplifiers
- Vibration transducers
- Nuclear instrumentation
[edit] Precautions
Cable from the transducer to the amplifier should not be subject to vibration as extra charges can be generated by friction and lead to unwanted noise output.
