SENSOR CIRCUITS

There are three types of amplifiers that are most often used in the design of sensor signal chains. Two of the more popular include the transresistance (transimpedance) amplifier and the charge amplifier. Additionally, there is the voltage amplifier which is often used with sensors that incorporate their own JFET. The transimpedance amplifier is used to convert a current to a voltage. Specifically, its gain can be specified in volts/A. A charge amplifier on the other hand is used to convert a charge to a voltage. A charge amplifier invariable uses a feedback capacitor to configure the amplifier as an integrator. The integrator effectively produces an output voltage that is proportional to the time rate of change of charge.

Transimpedance amplifiers, for the most part, are used in light or electromagnetic based sensor applications. Specifically, the current from a photodiode or a photodetector (induced from the light that strikes it) is converted to a voltage. On the other hand, charge amplifiers, are most often used with piezoelectric based sensors such as piezoelectric bases accelerometers, shock sensors, pressure sensors and hydrophones. However, it is still not uncommon to see piezoelectric sensors used with standard common source amplifiers. Photodiodes are also used with charge amplifiers. Often the selection of the amplifier is based on the characteristics of the sensor or detector used and the performance requirements of the system. Manufacturers will often integrate JFETs into their detectors or sensors to simplify the amplifier interface process. However, for high end performance applications, sophisticated JFET based preamps will be used to supplement the on-board JFET or the sensor will be purchased without the JFET.

TRANSIMPEDANCE AMPLIFIER SENSOR APPLICATIONS

The transimpedance amplifier is most often used in photodiode (photodetector) applications often used in a photo optical system. They are used across the electromagnetic frequency spectrum to determine the electromagnetic or light intensity that strikes a surface of the photodetector. This includes light in the gamma ray, X-ray, ultraviolet, visible light, near infrared, mid infrared and far infrared ranges.

The most simplistic topology of the transimpedance amplifier is shown in Figure 1. A current source provides the current that flows through the feedback resistor of the op-amp. This results in an output voltage that is directly proportional to the input current. The feedback resistor determines the transimpedance or transresistance gain, V/I.

A more real-life circuit is shown in Figure 2. It contains a feedback capacitor to prevent oscillations that are a result of parasitic capacitances. The feedback capacitor will affect the AC voltage gain and the stability of the circuit. Oscillations must be designed out.

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