Current-to-voltage converter
In electronics, a transimpedance amplifier (TIA) is a current-to-voltage converter, most often implemented using an operational amplifier. The TIA can be used to amplify the current output of Geiger–Müller tubes, photomultiplier tubes, accelerometers, photo detectors and other types of sensors to a usable voltage. Current-to-voltage converters are used with sensors that have a current response that is more linear than the voltage response. This is the case with photodiodes, where it is not uncommon for the current response to have better than 1% linearity over a wide range of light input. The transimpedance amplifier presents a low impedance to the photodiode and isolates it from the output voltage of the operational amplifier. In its simplest form a transimpedance amplifier has just a large-valued feedback resistor, Rf. The gain of the amplifier is set by this resistor and has a value of −Rf (because 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.
In the circuit shown above the photodiode is connected between ground and the inverting input of the op-amp. The other input of the op-amp is also connected to ground. This provides a low-impedance load for the photodiode, which keeps the photodiode voltage low. The photodiode is operating in photovoltaic mode with no external bias. The high gain of the op-amp keeps the photodiode current equal to the feedback current through Rf. The input offset voltage due to the photodiode is very low in this self-biased photovoltaic mode. This permits a large gain without any large output offset voltage. This configuration is used with photodiodes that are illuminated with low light levels and require a lot of gain.
The DC and low-frequency gain of a transimpedance amplifier is determined by the equation
so
If the gain is large, any input offset voltage at the non-inverting input of the op-amp will result in an output DC offset. An input bias current on the inverting terminal of the op-amp will similarly result in an output offset. To minimize these effects, transimpedance amplifiers are usually designed with FET input op-amps that have very low input offset voltages.
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