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Charge amplifier
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Charge amplifier
A charge amplifier is an electronic current integrator 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.
The charge amplifier was invented by Walter Kistler in 1950.
Charge amplifiers are usually constructed using an operational amplifier or other high gain semiconductor circuit with a negative feedback capacitor Cf.
Into the inverting node flow the input charge signal qin and the feedback charge qf from the output. According to Kirchhoff's circuit laws they compensate each other.
The input charge and the output voltage are proportional with inverted sign. The feedback capacitor Cf sets the amplification.
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.
The feedback resistor Rf discharges the capacitor. Without Rf the DC gain would be very high so that even the tiny DC input offset current of the operational amplifier would appear highly amplified at the output. Rf and Cf set the lower frequency limit of the charge amplifier.
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Charge amplifier
A charge amplifier is an electronic current integrator 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.
The charge amplifier was invented by Walter Kistler in 1950.
Charge amplifiers are usually constructed using an operational amplifier or other high gain semiconductor circuit with a negative feedback capacitor Cf.
Into the inverting node flow the input charge signal qin and the feedback charge qf from the output. According to Kirchhoff's circuit laws they compensate each other.
The input charge and the output voltage are proportional with inverted sign. The feedback capacitor Cf sets the amplification.
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.
The feedback resistor Rf discharges the capacitor. Without Rf the DC gain would be very high so that even the tiny DC input offset current of the operational amplifier would appear highly amplified at the output. Rf and Cf set the lower frequency limit of the charge amplifier.
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