Let there be light: Converting light into voltage

Converting light into a voltage is no trivial task. Photodiodes are used in various areas ranging from general purpose use, such as automatic faucets, hand dryers and bill counters, to industrial control and optical communication like encoders and optical receivers. Choosing the “right” amplifier depends on the application, but its selection must be carefully tailored to the diode itself.

That’s because the junction capacitance of the diode will form a pole with the feedback resistor of the amplifier to cause a phase lag. Compensating a rather simple configuration, like figure 1, should be a piece of cake right? Hmmm…well it may not be so straight forward.

Figure 1

Remember, the capacitance forming a pole with the op amp feedback resistor isn’t just from the diode. You must also account for parasitics, stray capacitance and the input capacitance of the op amp itself.

Then there’s the question of process differences. Should you pick a bipolar for low noise (voltage noise that is), a CMOS for low input bias current, or a JFET for both as well as low current noise? That choice depends on the sensitivity as well as the circuit components such as resistor values. Even though the input bias can drift quite a bit in a FET input device, it’ll likely do better up to 70ºC A 10pA Ib at 25ºC. The op amp will have just over 200pA at 85ºC which still beats any bipolar input. Remember, that the dark current will also exhibit the same effect (reversed biased diode or photoconductive mode).

For very low levels of currents, where you’ll need a rather large resistor, a JFET is probably a good starting point as it gives a good combination of low noises (voltage and current). One of my favorites is the OPA827.

In applications where offset voltage and drift are important, zero drift amplifiers such as the LMP2021 and OPA333 can be very good candidates. Many people claim that chopper stabilized amplifiers don’t make good transimpedance amplifiers partly because of their higher bias current yet they’re comparable to JFET’s at higher temperature.

In applications such as life sciences and analytical instruments (spectrometry and OTDR), higher speeds are usually needed. In these cases, selectable gain devices such as the OPA857 provide you more flexibility.

One last thing to remember is that decompensated op amps can be useful for your I-V conversion as long as you remember to manipulate the noise gain so that they are stable. For more info on decompensated op amps, check out my blog post called “decompensated op amps to the rescue.”

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