The OPA855 is decompensated and requires a high-frequency gain of 7V/V or greater to be stable. Using the OPA855 in lower gains results in increased peaking and potential instability. Decompensated amplifiers are advantageous in TIA applications due the inherent characteristics of a TIA design. The zero and pole pair introduced by the input and feedback capacitances along with the feedback resistor increases the noise gain until it flattens out at a high gain with a magnitude shown in Equation 2.

where

• C_TOT is the total input capacitance of the amplifier (includes photodetector capacitance and the common-mode and differential input capacitance of the amplifier)

• C_F is the feedback capacitance of the amplifier

A decompensated amplifier allows for benefits such as increased open loop gain, increased bandwidth, increased slew rate, and lower input referred noise for the same quiescent current relative to its unity gain stable counterpart.

Similar to the concept described in Section 10.2.2, the rise time and the internal capacitance of the photodetector will determine the closed-loop bandwidth. Both the closed-loop bandwidth and the transimpedance gain (R_F) determine the necessary gain bandwidth (GBWP) of the amplifier. Table 10-1 shows the standard photodiode characteristics based on type of photodetector. Target values such as the system bandwidth and gain were calculated using these concepts with the chosen photodiode characteristics. Detailed explanations and equations can be found in the application reports discussed in Section 10.2.2.

Figure 10-5 shows the OPA855 configured as a TIA, with the optical sensor reverse biased so that the diode cathode is tied to the positive bias voltage. A RC filter can be used at the reverse bias node as a low pass filter to eliminate high frequency noise. The internal capacitance of photodetectors will vary based on sensor type and the value of the applied reverse voltage. The setups between each sensor type will slightly differ, but the connection to the amplifier will be consistent throughout.

The difference between each optical design comprise choosing the optimal feedback resistor to set the transimpedance gain and the optimal feedback capacitance to compensate for the additional input capacitance. With an 8 GHz GBWP, the OPA855 can accommodate very fast rise times to pair with emerging optical sensors to meet the industry’s demands for faster optical detections.

The DC voltage bias at the non-inverting input of the OPA855 shown in Figure 10-5 will set the common-mode voltage which will maximize the output swing of the system in mismatched power supply configurations. The DC bias is critical to avoid clipping or saturating the output stage of the amplifier. For the later stages, a fully differential amplifier (FDA) can be used to convert single-ended signal to a differential input to drive an analog-to-digital converter (ADC) as shown in Figure 10-1. Higher order filters can be added between the FDA and ADC for system noise reduction.

Figure 10-7 shows the performance that results from the design parameters provided in Table 10-1, and Figure 10-8 shows the general trends. Both figures depict the closed-loop bandwidth performance of the OPA855 configured as a TIA using different sensor types and gain configurations. Figure 10-7 shows the amplifier performance based on the chosen photodetector from the values provided in Table 10-1. PMTs and MPPCs have higher intrinsic gains, but requires a wide bandwidth to compensate for its higher internal capacitance. Whereas, PDs and APDs require higher gain configurations to achieve similar output voltage levels. The OPA855 is able to provide the bandwidth to accommodate for both optical challenges. Figure 10-7 shows a generic view of the amplifier performance as a function of sensor capacitance and transimpedance gain. Increasing the feedback resistance and input capacitance, decreases the closed-loop bandwidth. Throughout the trends, the amount of change in closed-loop bandwidth is consistent in relationship of the changes in both terms. A photodiode capacitance of 1 pF and a feedback resistance of 1 kΩ results in a very high closed-loop system bandwidth of 1.1 GHz.