Closed-loop TIA Bandwidth (f-3dB)

24.00

MHz

Feedback Resistance (RF)

100.00

kOhm

Input Capacitance (CIN)

7.40

pF

Opamp Gain Bandwidth Product (GBP)

2678.14

MHz

Feedback Capacitance (CF)

0.092

pF

Calculator II

VO

+5 V

±5 V

+

±

RF

100 k

C1

2.2 pF

VBIAS

OPA818

Supply decoupling

not shown

CD

5 pF

C2

2.2 pF

CT

47 pF

CDIFF

0.5 pF

CCM

1.9 pF

OPA818's input differential and

common-mode capacitance

21

OPA818

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SBOS940A – MAY 2019 – REVISED MARCH 2020

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9.2 Typical Applications

9.2.1 High Bandwidth, 100-kΩ Gain Transimpedance Design

The high GBWP and low input voltage and current noise for the OPA818 make it an excellent wideband

transimpedance amplifier for moderate to high transimpedance gains.

Figure 51. Wideband, High-Sensitivity, Transimpedance Amplifier

9.2.1.1 Design Requirements

Design a high-bandwidth, high-transimpedance-gain amplifier with the design requirements shown in Table 1.

Table 1. Design Requirements

TARGET BANDWIDTH (MHz)

TRANSIMPEDANCE GAIN (kΩ)

PHOTODIODE CAPACITANCE (pF)

24

100

5

9.2.1.2 Detailed Design Procedure

Designs that require high bandwidth from a large area detector with relatively high transimpedance gain benefit

from the low input voltage noise of the OPA818. This input voltage noise is peaked up over frequency by the

diode source capacitance, and can, in many cases, become the limiting factor to input sensitivity. Figure 51

shows the transimpedance circuit with the parameters as defined in Design Requirements. To use the Microsoft

Excel™ calculator available at What You Need To Know About Transimpedance Amplifiers – Part 1 to help with

the component selection, total input capacitance, CTOT, needs to be determined. CTOT is referred as CIN in the

calculator. CTOT is the sum of CD, CDIFF, and CCM which is 7.4 pF. Using this value of CTOT, and the targeted

closed-loop bandwidth (f–3dB) of 24 MHz and transimpedance gain of 100 kΩ results in a need for an amplifier

with approximately 2.68 GHz GBWP and a feedback capacitance (CF) of 0.092 pF as shown in Figure 52. These

results are for a Butterworth response with a Q = 0.707 and a phase margin of approximately 65° which

corresponds to 4.3% overshoot.

Figure 52. Results of Inputting Design Parameters in the TIA Calculator