A short primer on OTRA and OTIA amplifier devices
Article By : Jean-Jaques (JJ) DeLisle
OTRAs/OTIAs perform current-to-voltage conversion, much like a resistor, but they exhibit enhanced input and output characteristics.
Operational transresistance amplifiers (OTRAs), also known as operational transimpedance amplifiers (OTIAs), have grown in use since the late 1980s. Mainly, the value of OTRAs/OTIAs is for current-to-voltage conversion, much like a resistor. However, an OTRA/OTIA device exhibits enhanced input and output characteristics, even though these circuits exhibit the same effective gain as a resistor.
OTRA/OTIAs are current feedback operational amplifiers (CFOAs) that are often used to convert the current output of sensors to usable voltages (Figure 1). Examples of sensors with current outputs that are more linear or usable than their voltage outputs are photodiodes/photodetectors, accelerometers, photo-multiplier tubes, and Geiger-Muller tubes. In essence, an OTRA/OTIA presents a very low impedance at the input while isolating the input node from the voltage at the output of the amplifier circuit.
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(b)
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Figure 1 The above diagrams show topology of a transresistance amplifier (a), high level diagram symbol of an operational transresistance amplifier (b), and the equation matrix of an operational transresistance amplifier (c). Source: Springer
OTRA/OTIA topology
A simple topology of an OTRA/OTIA device embodies a single feedback resistor alongside the non-inverting input grounded. As no current enters the inverting input of the op-amp, all the current from the inverting input flows through the resistor R2, developing a voltage difference between the inverting node input and the output node. As the noninverting input is tied to ground, the inverting input voltage must also be at ground. This results in the voltage difference with a negative output voltage referenced to the virtual ground at the inverting input.
With a typical operational amplifier, the input impedance is very high, which is desirable and leads to more effective voltage signal conversion. However, with an OTRA/OTIA, it’s desirable to have as low an impedance as possible, as these circuits are current-mode devices. This is the case with OTRAs/OTIAs as the inverting input terminal is a virtual ground and hence has a very low input impedance, essentially zero, even when large feedback resistors are used to achieve high gain.
This response is unlike a passive resistor circuit, which requires a high input impedance to achieve a high gain. Moreover, a passive resistor circuit forms a parallel resistance with the load resistance, which results in a gain that is dependent on the resistance equivalent with the load. An OTRA/OTIA, on the other hand, exhibits low output impedance as well.
The behavior of OTRAs/OTIAs is also different from typical voltage feedback operational amplifiers (VFOAs), as OTRAs/OTIAs do not have balanced inputs as VFOAs do. OTRAs/OTIAs exhibit high impedance at the non-inverting input, but low impedance at the inverting input. Additionally, OTRAs/OTIAs have an open-loop gain expressed in Ohms, which is a transimpedance, instead of V/V as is the case with VFOAs.
The basic circuit for open loop gain for an OTRA/OTIA is depicted in Figure 2. The closed loop gain of a OTRA/OTIA with a fixed value feedback resistor (R2) can be adjusted by changing R1. This change doesn’t significantly impact the closed-loop bandwidth of the op-amp. This response allows for the relatively straightforward realization of a variable gain transimpedance amplifier circuit.
Figure 2 Here is a view of current feedback operational amplifier, transresistance operation amplifier, or transimpedance operational amplifier topology. Source: Analog Devices
OTRA/OTIA recommendations
There is a recommended value of feedback resistor for a given OTRA/OTIA that allows for maximum bandwidth. Operating an OTRA/OTIA with a resistance outside of the optimum value may result in instability and reduce overall bandwidth, but may be compensated for using capacitance parallel with the feedback resistor. It’s generally inadvisable for the inverting input to be directly connected to the output of an OTRA/OTIAs, as this results in an extremely low feedback resistance and will likely result in instability.
In most cases it’s desirable to minimize the capacitance seen at the inverting input and eliminate capacitance from the feedback loop. Capacitance in the feedback loop reduces the feedback impedance at higher frequencies and may induce oscillations at higher frequencies. Capacitance at the inverting input causes a similar effect, which is problematic in some circuits, such as photodiodes, which exhibit a parasitic capacitance.
Moreover, the slew rate of an OTRA/OTIA device is only limited by the internal parasitic capacitances. With good design, OTRAs/OTIAs tend to exhibit high bandwidth and good distortion performance compared to VFOAs.
This article was originally published on Planet Analog.
Jean-Jaques (JJ) DeLisle, an electrical engineering graduate (MS) from Rochester Institute of Technology, has a diverse background in analog and RF R&D, as well as technical writing/editing for design engineering publications. He writes about analog and RF for Planet Analog.