Transimpedance amplifier

In electronics, a transimpedance amplifier (TIA) is a current to voltage converter, almost exclusively implemented with one or more operational amplifiers (opamps). The TIA can be used to amplify the current output of Geiger–Müller tubes, photo multiplier tubes, accelerometers, photodetectors and other sensors (that are modeled well as a current source) into a usable voltage.

Current to voltage converters are used with sensors that have a current response to their measured physical quantity that is more linear than their voltage response. This is the case with photodiodes where it is not uncommon for the current response to have better than 1% nonlinearity over a wide range of light input.

The transimpedance amplifier presents a low impedance to the sensor and isolates it from the output voltage of the operational amplifier. In its simplest form (Fig. 1), a transimpedance amplifier is just an opamp with a large-valued feedback resistor, Rf. This resistor sets the amplifier's transimpedance (i.e. its change in output voltage divided by its change in input current, sometimes simply referred to as "gain") to -Rf. This is negative since the amplifier is in an inverting configuration.

There are several different configurations of transimpedance amplifiers, each suited to a particular application. The one factor they all have in common is the requirement to convert the low-level current of a sensor to a voltage. The gain, bandwidth, as well as current and voltage offsets change with different types of sensors, requiring different configurations of transimpedance amplifiers.

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