Charge amplifier
Encyclopedia
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.
Common applications include piezoelectric sensor
s and photodiode
s, 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:
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).
Charge Transfer Amplifier
Piezoelectric sensor
A piezoelectric sensor is a device that uses the piezoelectric effect to measure pressure, acceleration, strain or force by converting them to an electrical charge.-Applications:...
. Contrary to what its name may suggest, a charge amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
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
Miller effect
In electronics, the Miller effect accounts for the increase in the equivalent input capacitance of an inverting voltage amplifier due to amplification of the effect of capacitance between the input and output terminals...
. 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.
Common applications include piezoelectric sensor
Piezoelectric sensor
A piezoelectric sensor is a device that uses the piezoelectric effect to measure pressure, acceleration, strain or force by converting them to an electrical charge.-Applications:...
s and photodiode
Photodiode
A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation.The common, traditional solar cell used to generateelectric solar power is a large area photodiode....
s, in which the charge output from the transducer is converted into a voltage. Charge amplifiers are often found in instrumentation
Instrumentation
Instrumentation is defined as the art and science of measurement and control of process variables within a production, or manufacturing area....
, and in the readout circuitry of CCD
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
imagers and flat-panel X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
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:
- 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
Design
Charge amplifiers are usually constructed using op-amps with a feedbackFeedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
capacitor. Since the transducer acts in a similar manner to a differentiator
Differentiator
A Differentiator is a circuit that is designed such that the output of the circuit is proportional to the time derivative of the input. There are two types of differentiator circuits, active and passive.-Theory:...
, 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
Virtual ground
Virtual ground is a node of the circuit that is maintained at a steady reference potential, without being connected directly to the reference potential...
(looking from the side of the input source, the circuit has zero input resistance).
Applications
- AccelerometerAccelerometerAn accelerometer is a device that measures proper acceleration, also called the four-acceleration. This is not necessarily the same as the coordinate acceleration , but is rather the type of acceleration associated with the phenomenon of weight experienced by a test mass that resides in the frame...
signal conditioningSignal conditioningIn electronics, signal conditioning means manipulating an analog signal in such a way that it meets the requirements of the next stage for further processing... - Guitar pickup amplifiers
- VibrationOscillationOscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...
transducers - NuclearAtomic nucleusThe nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
instrumentationInstrumentationInstrumentation is defined as the art and science of measurement and control of process variables within a production, or manufacturing area....
See also
Obtaining virtual zero impedance by applying Miller theoremCharge Transfer Amplifier
Charge transfer amplifier
The charge-transfer amplifier is an electronic amplifier circuit. Also known as transconveyance amplifiers, CTAs amplify electronic signals by dynamically conveying charge between capacitive nodes in proportion to the size of a differential input voltage...
External links
- Interface Circuits for Polyvinylidene fluoridePolyvinylidene fluoridePolyvinylidene fluoride, or PVDF is a highly non-reactive and pure thermoplastic fluoropolymer.PVDF is a specialty plastic material in the fluoropolymer family; it is used generally in applications requiring the highest purity, strength, and resistance to solvents, acids, bases and heat and low...
film - Schematic diagram
