Transimpedance amplifiers (TIAs) are used to convert an input current into an output voltage.

Applications

Transimpedance amplifiers are useful in many important applications, including:

• Geiger–Müller tubes
• photomultiplier tubes
• accelerometers
• photo detectors
• other types of sensors

A common use of TIAs is in receivers for optical communication. There are several different configurations of TIAs, each suited to a particular application. Current-to-voltage conversion is achieved by a feedback resistor stabilized by a sufficiently-large capacitor.

TIAs can operate as both active and passive current-to-voltage converters. Passive converters use only passive components, as the name implies. A simple example is a resistor in a series with a current source and a resistor. Voltage measured across the resistor can be converted to current using Ohm's law (V=IR.) Active TIAs use active elements such as FETs, BJTs, and other transistors. Most commonly, an operational amplifier is used in the TIA to achieve current-to-voltage conversion.

Specifications

When specifying TIAs, there are several important values to consider. Operating temperature is specified by level of the ambient temperature (in °C) in which the amplifier was designed to operate. Data rate is the number of bits per second that the device is capable of moving. Transimpedance is the resistance measured from input terminals to output terminals. Power dissipation (PD) is the maximum dissipation (in watts) that the amplifier can support safely. Supply voltage (VS) refers to the source voltage range. Supply current (I_S) is the current produced by the supply source when connected to the amplifier.  Bandwidth refers to the device's ability to provide a maximum output voltage swing with increasing frequency. At certain frequencies the output becomes slew-rate limited and begins to degrade. This frequency is the upper limit of the power bandwidth. The output voltage at this frequency is the peak output swing of the amplifier. Open-loop gain (AVOL) is the maximum amplifier gain when connected in an open loop.

Features

TIAs can be configured in any one of a number of package types. These include: DIP, CDIP, PDIP, DPAK, CSP, SIP, SOIC, SSOP, SOP, MSOP, and others. Features available on a TIA to customize for a particular application might be bidirectional current sense amplifiers that can conduct current in both directions. Also, an integrated reference where the comparator component has its own embedded reference voltage. Thermal shutdown protection occurs when the regulator has an embedded control circuit that shuts down the device once the temperature exceeds a predefined limit. Radiation protection is another feature that is available that may be necessary by spec or application.

TIAs are essential building blocks in any system that measures light. Many material analysis apparatus rely on photodiodes to accurately detect compounds. Such systems find it essential to measure a range of light intensities. Some substances have strong absorption bands at certain wavelengths, while remaining almost transparent at other wavelengths. Instrumentation systems that measure physical properties using a photodiode often include a TIA to maximize dynamic range and minimize noise while maintaining high bandwidth and high accuracy.

Related Information

CR4 Community—Transimpedance Amplifier Bandwidth Calculation

Engineering360—Fiber Optic Receivers Information

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Texas Instruments