A charge amplifier is an electronic currentintegrator that produces a voltage output proportional to the integrated value of the input current, or the total charge injected. The amplifier offsets the input current using a feedback reference capacitor, and produces an output voltage inversely proportional to the value of the reference capacitor but proportional to the total input charge flowing during the specified time period. The circuit therefore acts as a charge-to-voltage converter. The gain of the circuit depends on the values of the feedback capacitor.
Design
Charge amplifiers are usually constructed using an operational amplifier or other high gain semiconductor circuit with a negative feedback capacitor. The input current is offset by a negative feedback current flowing in the capacitor, which is generated by an increase in output voltage of the amplifier. The output voltage is therefore dependent on the value of input current it has to offset and the inverse of the value of the feedback capacitor. The greater the capacitor value, the less output voltage has to be generated to produce a particular feedback current. The input impedance of the circuit is almost zero because of the Miller effect. Hence all the stray capacitances are virtually grounded and they have no influence on the output signal.
Ideal circuit
The circuit operates by passing a current through a capacitor Cf. The opamp strives to retain the virtual ground condition at the input by off-setting the effect of the input current. If the op-amp is assumed to be ideal, nodes v1 and v2 are held equal, and so v2 is a virtual ground. The opamp generates a compensating current flow through the series capacitor to maintain the virtual ground. This charges or discharges the capacitor over time. Because the capacitor is connected to a virtual ground, the input current does not vary with capacitor charge and a linear integration of output is achieved. The relationship between electric charge and current is governed by: Therefore, the input-output equation of the charge amplifier is:
Applications
Common applications include amplification of signals from devices such as piezoelectric sensors and photodiodes, in which the charge output from the device is converted into a voltage. Charge amplifiers are also used extensively in instruments measuring ionizing radiation, such as the proportional counter or the scintillation counter, where the energy of each pulse of detected radiation due to an ionising event must be measured. Integrating the charge pulses from the detector gives a translation of input pulse energy to a peak voltage output, which can then be measured for each pulse. Normally this then goes to discrimination circuits or a multi channel analyzer. Charge amplifiers are also used in the readout circuitry of CCD imagers and flat-panel X-ray detector arrays. The amplifier is able to convert the very small charge stored within an in-pixel capacitor to a voltage level that can be easily processed. Advantages of charge amplifiers include:
Enables quasi-static measurements in certain situations, such as constant pressures on a piezo lasting several minutes
Piezo element transducer can be used in much hotter environments than those with internal electronics
Gain is dependent only on the feedback capacitor, unlike voltage amplifiers, which are affected greatly by the input capacitance of the amplifier and the parallel capacitance of the cable
