9.2.2 Detailed Design Procedure

The OPA856 meets the growing demand for wideband, low-noise photodiode amplifiers. The closed-loop

bandwidth of a transimpedance amplifier is a function of the following:

amplifier (common-mode and differential capacitance) and any stray capacitance from the PCB.

2. The op amp gain bandwidth product (GBWP).

Figure 9-1 shows the OPA856 configured as a TIA, with the photodiode reverse biased so that the diode cathode

is tied to a positive bias voltage. In this configuration, the diode sources current into the op amp feedback loop

so that the output swings in a negative direction relative to the input common-mode (VOFFSET) voltage. The

left unchecked. To counteract the effect of the zero, a pole is inserted into the noise gain transfer function by

The Transimpedance Considerations for High-Speed Amplifiers Application Report discusses theories and

equations that show how to compensate a transimpedance amplifier for a particular transimpedance gain and

input capacitance. The bandwidth and compensation equations from the application report are available in a

Microsoft Excel ™ calculator. What You Need To Know About Transimpedance Amplifiers – Part 1 provides a

link to the calculator. Calculating the expected bandwidth with an approximate input capacitance of 4 pF and a

feedback capacitor of 1 pF yields a bandwidth of approximately 200 MHz.

The amplifier was tested in a transimpedance configuration by using a photodiode with an optical fiber input

connection. A tunable laser connected through an optical modulator was used to create the modulated optical

excitation to the photodiode. Figure 9-2 shows the test setup configuration for the frequency response

measurement. The network analyzer's swept frequency output drives the optical modulators electrical input

which in turn drives the photodiode. The OPA856 output drives the network analyzer's input.

+

±

1 NŸ

5V

ON

OFF

1 pF

169

71.5

1330 nm

-

-

-

Optical

Modulator

Tunable Laser

Network Analyzer

OPA856

200-Ÿ ORDG PDWFKHG

to 50-Ÿ LQSXW

Fiber connected

photodiode

Figure 9-2. OPA856 Transimpedance Frequency Response Test Setup

Figure 9-4 shows the frequency response measurements for a small signal and large signal (~1 Vpp) output.

The plot contains noticeable noise and variations because the test environment did not have complete capability

to accurately manage the thermal drift, perform optical connection integrity analysis, and calibrate the optical

path. A more stringently controlled optical environment could achieve more stable results, but was beyond the

scope of these measurements. The results in Figure 9-4 correlate well with predicted results of approximately

200 MHz of bandwidth. It is expected that the results would not perfectly match calculated values because it is

challenging to perfectly account for all parasitic capacitances that affect the input and feedback capacitance in

the transimpedance calculations.

OPA856

SBOS623 – OCTOBER 2020

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