A charge amplifier is a current integrator driven by an electrical source with capacitive nature such as a piezoelectric sensor. Contrary to what its name may suggest, a charge amplifier does not amplify the electric charge present at its input (it can amplify only the exciting input voltage). The charge amplifier just transfers the input charge to another reference capacitor and produces an output voltage equal to the voltage across the reference capacitor. Thus the output voltage is proportional to the charge of the reference capacitor and, respectively, to the input charge; hence the circuit acts as a charge-to-voltage converter. The input impedance of the circuit is almost zero because of the Miller effect. Hence all the stray capacitances (the cable capacitance, the amplifier input capacitance, etc) are virtually grounded and they have no influence on the output signal.[1]
Common applications include piezoelectric sensors and photodiodes, in which the charge output from the transducer is converted 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.
Advantages include:
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Charge amplifiers are usually constructed using op-amps with a feedback capacitor. 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 (looking from the side of the input source, the circuit has zero input resistance).
Obtaining virtual zero impedance by applying Miller theorem
