8.1.2 Wideband, Transimpedance Design Using the OPA818

With high GBWP, low input voltage and current noise, and low input capacitance, the OPA818 design is

optimized for wideband, low-noise transimpedance applications. The high-voltage capability allows greater

flexibility of supply voltages along with wider output voltage swings. Figure 8-2 shows an example circuit of

a typical photodiode amplifier circuit. Generally the photodiode is reverse biased in a TIA application so the

photodiode current in the circuit of Figure 8-2 flows into the op amp feedback loop resulting in an output voltage

that reduces from VREF with increasing photodiode current. In this type of configuration and depending on the

application needs, VREF can be biased closer to VS+ to achieve the desired output swing. Do not violate the input

common-mode range requirements when the VREF bias is used.

The key design elements that determine the closed-loop bandwidth, f–3dB, of the circuit are:

1. The op amp GBWP

2. The transimpedance gain, RF

3. The total input capacitance, CTOT, that includes photodiode capacitance, input capacitance of the amplifier

(common-mode and differential capacitance), and PCB parasitic capacitance

V

O

+5 V

–5 V

+

–

R

F

C

F

V

BIAS

OPA818

V

REF

Figure 8-2. Wideband, Low-Noise, Transimpedance Amplifier

Equation 1 shows the relationship between the previously mentioned three elements for a Butterworth response.

f–3dB =  GBWP

2πRFCTOT

(1)

The feedback resistance, RF and the total input capacitance, CTOT cause a zero in the noise gain that results

in instability if left uncompensated. To counteract the effect of the zero, a pole is inserted in the noise gain by

adding the feedback capacitor, CF. The Transimpedance Considerations for High-Speed Amplifiers application

report discusses theories and equations that show how to compensate a transimpedance amplifier for a

particular gain and input capacitance. The bandwidth and compensation equations from the application report

are available in an Excel™ calculator. What You Need To Know About Transimpedance Amplifiers – Part 1

provides a link to the calculator. The details of maximizing the dynamic range of TIA front-ends (see High-Speed

Optical Front-End) that uses voltages VREF1 and VREF2 are provided in the Maximizing the dynamic range of

analog TIA front-end application note.

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OPA818

SBOS940B – MAY 2019 – REVISED DECEMBER 2025

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