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Overcoming common challenges driving an amplifier with a photodiode

Optical systems have many unique attributes that make them useful for range detection and communications, but often the optical signal needs to be converted to an electrical signal as part of the overall system design. A photodiode is an excellent optical-to-electrical transducer, but it is rarely able to create a signal large or powerful enough to be useful.

A transimpedance amplifier (TIA), which takes the current from a photodiode and converts it into a usable voltage with low output impedance, can provide effective gain for a photodiode signal. Optimizing the photodiode-to-TIA interface is critical for system performance. This article will cover some basic requirements when designing a circuit around a photodiode.

One of the first challenges is establishing the proper bias voltage for the photodiode. While it is possible to use a photodiode as a quasi-solar cell (that is, no external bias voltage), nearly every application benefits from an external bias voltage applied to the photodiode. Review the photodiode data sheet and look for charts that show junction capacitance and responsivity with respect to reverse-bias voltage. Choose the lowest voltage that meets your system requirements, and then add at least 10% for process variations in the photodiode manufacturing process.

When using avalanche photodiodes, you will most likely have a bias voltage that is significantly larger than the amplifier’s supply voltage, and also much higher than the amplifier’s input-voltage range. In this case, make sure that your supply sequencing does not produce a voltage surge on the amplifier input during power up. Use both system modeling and bench measurements to confirm that you are not damaging the amplifier’s input stage.

Many photodiode applications, such as light detection and ranging (LIDAR) or digital communications, require a very fast response from the photodiode. Photodiodes with high bandwidth will have very low junction capacitance: lower than 1 pF. In order to preserve this fast response, very carefully design the circuit board connecting the photodiode to the amplifier. Because the photodiode usually has a high output impedance, you cannot use transmission lines to connect the photodiode and the amplifier; they must be located as close as possible to one another. In fact, some optical communications modules are built with the photodiode glued to the top of the TIA die, as shown in Figure 1.

Figure 1

Photodiode die mounted Directly on a TIA die

Photodiode die mounted Directly on a TIA die

Photodiode die mounted Directly on a TIA die

The PC board layout shown in Figure 2 is useful in illustrating the use of separately packaged devices. While the TIA is very small (2mm by 2 mm), the photodiode is mounted in a transistor-outline-sized metal canister that is much larger. The size of the photodiode package makes it harder to locate the amplifier close to the photodiode. Also, the TIA and photodiode require passive components, including a feedback resistor, feedback capacitor, and supply- and bias-decoupling capacitors. The diode on the board shown in Figure 2 is approximately 5 mm from the TIA. The trace that connects the diode to the TIA is very thin to reduce capacitance on the trace. The trace width is a compromise, however, because a thin trace adds undesirable inductance. For circuits with very high bandwidth (over 500 MHz), consider using a high-performance printed circuit board dielectric. The most common board material, FR4, has high loss and poorly controlled impedance at high frequencies.

Treat the photodiode bias voltage like the power supply for a high-speed amplifier. There should be high-frequency bypass capacitors located close to the diode cathode (photodiodes are reverse-biased in most applications). The bypass capacitor will supply high-frequency current to the diode, which will in turn drive the TIA input.

The TIA input pin will be very sensitive to capacitance to ground, so minimize it when you lay out the PCB. In Figure 2’s layout, the only connections to the TIA input pin are the feedback resistor and feedback capacitor. The components shown in Figure 2 are a 0402-size resistor and a 0201-size capacitor.

Figure 2

EVM layout showing Diode mounted close to TIA

EVM layout showing Diode mounted close to TIA

EVM layout showing Diode mounted close to TIA

One fact about a photodiode as a signal source, that may not be immediately obvious, is that the current from a photodiode flows in only one direction; in other words, it is unipolar. In the circuit shown in Figure 3, you can compensate for this behavior by adjusting the TIA power-supply voltages so that the quiescent state is biased well above the mid-supply voltage. This sets the no-light condition to be near the amplifier’s maximum output voltage. As the illumination on the photodiode increases and the photodiode current increases, the amplifier voltage will swing more negative.

The value of the feedback resistor will determine the amplifier output voltage when the diode illumination is at the maximum value. If the feedback resistor is too large, the amplifier output will saturate at a photodiode illumination level that is below maximum. The compensation capacitor (C1) will help prevent overshoot of the output signal caused by the photodiode’s junction capacitance.

Equation 1 expresses the predicted value of the compensation capacitor. Plan on adjusting this capacitor value once you have built a prototype circuit, because simulation results are usually not accurate enough to select the optimum value, especially when the feedback capacitor is very small (<5 pF). This is illustrated in Figure 3, where the feedback capacitor is calculated as 116 fF, as shown in Equation 2. A simulation showed optimum performance with a 175-fF feedback capacitor, but a prototype showed that board parasitic capacitance was compromising performance, which made a board redesign necessary.

Figure 3

Simplified Schematic of a TIA circuit.

Simplified Schematic of a TIA circuit.

When designing a photoelectric circuit, it is very important to follow best practices when connecting the photodiode to the TIA. Following the simple steps outlined here will help ensure high-speed signal integrity.

References

AN-1803 Design Considerations for a Transimpedance Amplifier.”

Photodiode Amplifier Circuit.

1 comment on “Overcoming common challenges driving an amplifier with a photodiode

  1. Tucson_Mike
    February 4, 2019

    Hey Loren, nice post -yea the first pass at getting Cf=0.175pF seemed a low. 

    I am guessing the reason the original calculation for Cf was too low was the unity gain closed loop bandwidth of 1.8GHz was used in Eq. 2 where that should have been the GBP, more like 900MHz – Eq. 1 is implicitly solving to place the feedback pole at Fo. that will give a Q=1 and likely the final Cf shown was slightly increased to empirically reduce the peaking (1.25dB)  from that condition. 

    I touched on these issues a bit in a new blog showing up on PLanet Analog, Feb. 3, 2019. 

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