9.1.2 Wideband, Transimpedance Design Using the OPA814

The OPA814 design is optimized for wideband, low-noise transimpedance applications with high GBWP,

low input voltage, low current noise, and low input capacitance. The high-voltage capability allows greater

flexibility of supply voltages along with wider output voltage swings. Figure 9-3 shows an example circuit of a

typical photodiode amplifier circuit. Figure 9-3 shows that the photodiode is generally reverse biased in a TIA

application, so that the photodiode current in the circuit flows into the op-amp feedback path. This polarity of the

current results 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. Consider the common-mode input range when VREF bias is used so that the common-mode input

voltage stays within the valid range of the OPA814.

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

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

OPA814

V

REF

C

TOT

Figure 9-3. Wideband, Low-Noise, Transimpedance Amplifier

Equation 1 shows the relationship between the three key design elements for a Butterworth response.

f–3dB=  GBWP

2×π×RF×CTOT

(1)

The feedback resistance (RF) and the total input capacitance (CTOT) form a zero in the noise gain, and

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 a Microsoft Excel™ calculator. A link to the calculator is provided in What You Need To

Know About Transimpedance Amplifiers – Part 1. The details of maximizing the dynamic range of TIA front-ends

are provided in the Maximizing the Dynamic Range of Analog TIA Front-End application note.

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OPA814

SBOSA14 – APRIL 2023

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