Transimpedance Amplifier Selection and Applications

Ready to convert current to voltage? You’ll need a transimpedance amplifier.

Everyone reading this probably remembers hearing about operational amplifiers in their electronics 101 classes, but their importance doesn’t always become obvious until you start using them for various applications. There are a number of different implementations for operational amplifiers, each of which is given its own special name. In particular, current-to-voltage conversion might seem as simple as requiring the use of a resistor and Ohm’s law. However, the reality is a bit more complex than using a resistor in parallel with a load.

Transimpedance amplifiers provide exactly this functionality, which makes it possible to readout a current from a device like a photodiode or a transceiver as a voltage, which can then be converted to a digital signal. While you might do something similar with another type of amplifier and some external components, you can save space on your board and get access to some other features when you use a transimpedance amplifier IC. Here are some options for your next system.

What is a Transimpedance Amplifier?

One thing that confused me in my younger years was what exactly makes various amplifiers different. If you look at a circuit diagram for a transimpedance amplifier, it looks quite similar to an operation amplifier circuit with negative feedback. So what makes it different from an operational amplifier? The answer is: you can build a transimpedance amplifier from an operational amplifier; the difference is in the signal being fed into the amplifier circuit and how feedback works within the circuit.

Rather than going through all the theory involved in transimpedance amplifiers, the point of all this is that you can use a transimpedance amplifier to convert an input current into a voltage. This is important in many applications, such as:

• Photodiodes and optical equipment: These components output current, but this needs to be converted to a digital signal using an ADC. The transimpedance amplifier stage converts this current into a voltage before being input into an ADC. One upcoming area is in lidar systems for autonomous vehicles.

• Low-power analog sensors: Signals from pressure transducers, accelerometers, and other components that output a current can be converted to a voltage and fed to an ADC.

• RF equipment: Telecom and scientific applications make use of transimpedance amplifiers operating at microwave frequencies.

This circuit diagram shows the typical op-amp connection used to build an uncompensated transimpedance amplifier.

If you’re designing for one of these applications, you might opt for a transimpedance amplifier IC, rather than grabbing an op-amp IC and configuring it as a transimpedance amplifier. These ICs are optimized for specific applications and include other features that may be difficult to design with discrete components.

Important Specifications

Some important transimpedance amplifier specifications are as follows:

• Transfer impedance. This is equivalent to the gain of the amplifier. The transfer impedance multiplied by the input current gives the output voltage.

• Transimpedance bandwidth. All transimpedance amplifiers have low-pass transfer functions when operating in the linear range. Many practical applications deal with digital or pulsed currents, and the bandwidth of the pulse should not exceed the amplifier’s input bandwidth. This specification has the same meaning as unity-gain bandwidth, i.e., increasing the bandwidth requires decreasing the gain.

• Linear range. Like any other op-amp, a transimpedance amplifier IC can saturate when the input signal is very large. Range may be specified as an upper limit or as a lower limit and some dynamic range in dB.

• Compensation. This is important in components being used for photodiode sensing or any other component with parasitic capacitance. Because of the parasitic capacitance in a photodiode’s circuit model, a resonance can arise in the transimpedance amplifier’s transfer function. This can be seen in a gain vs. input frequency plot, where different curves will arise for different values of the source component’s parasitic capacitance. An amplifier with internal compensation allows use of a source component with higher parasitic capacitance.

• Referred RMS current noise. This tells you the RMS noise power spectral density (in current terms) in closed loop operation. This will be a function of gain in the negative feedback loop. High-quality components will have ~1-10 pA/√Hz RMS current, which translates into 1-10 mV noise on the output signal for a 100 MHz bandwidth at gain of 10,000.

Maxim Integrated, MAX40662

The MAX40662 transimpedance amplifier from Maxim Integrated is a quad-channel device designed for optical distance measurements in lidar receivers and related applications involving current pulses. The transfer impedance in this component is pin-selectable (25 and 50 kOhm) with very low noise (2.1 pA/√Hz power spectral density), making this component ideal for fast pulsed current measurements with low jitter. It also includes an internal multiplexer, and bandwidth is rated up to 440 MHz, which will easily support 10 ns current pulses.

MAX40662 transimpedance amplifier application circuit. From the MAX40662 datasheet.

Texas Instruments, LMH32401IRGTT

The LMH32401IRGTT from Texas Instruments is ideal for operation in noisy environments thanks to its differential output. The output gain has 2 settings while still providing a high gain-bandwidth product (rated up to 275 MHz at 20 kOhm, or 450 MHz at 2 kOhm). Ideal applications for this component include computer vision, mechanically-scanned lidar, time-of-flight position measurements, and related applications involving pulsed current sources.

For electro-optical measurement applications, this transimpedance amplifier includes an integrated ambient light cancellation circuit and 100 mA current clamp circuit to damp transients. At the higher gain setting, this component can sense current pulses as short as 800 ps. Input noise is also referenced to 49 nA RMS at full bandwidth, providing wide dynamic range for current measurements.

Block diagram and transimpedance bandwidth at each gain setting. From the LMH32401 datasheet.

Analog Devices, HMC799LP3E

The HMC799LP3E transimpedance amplifier from Analog Devices is intended for RF applications like IF-to-HF upconversion. Transfer impedance of 10 kOhm with 700 MHz bandwidth is available with high dynamic range of 65 dB. The output is internally impedance matched to 50 Ohms, making this component compatible with other components found in typical RF systems.

HMC799LP3E transimpedance amplifier functional diagram and transfer impedance. From the HMC799LP3E datasheet.

These are just some of the component options you’ll find on the market, and many others are specialized for functions outside of electro-optics. Although the above components have been marketed for use in electro-optical applications, they can be used with a range of other analog sensors.

Whether you need a transimpedance amplifier or other specialty-purpose amplifier, you can find exactly the components you need when you use the advanced search and filtration features in Octopart. You’ll have access to an extensive search engine with distributor data and parts specifications, all of which is accessible in a user-friendly interface. Take a look at our linear integrated circuits page to find the components you need for signal acquisition, amplification, and filtering.

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