8.2 Typical Applications

8.2.1 High-Bandwidth, 100-kΩ Gain Transimpedance Design

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

transimpedance amplifier for moderate-to-high transimpedance gains.

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 Input Differential and

Common-Mode Capacitance

Figure 8-3. Wideband, High-Sensitivity, Transimpedance Amplifier

8.2.1.1 Design Requirements

Design a high-bandwidth, high-transimpedance-gain amplifier with the design requirements shown in Table 8-1.

Table 8-1. Design Requirements

TARGET BANDWIDTH

(MHz)

TRANSIMPEDANCE GAIN

(kΩ)

PHOTODIODE CAPACITANCE

(pF)

24

100

5

8.2.1.2 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 in many cases, becomes the limiting factor to input sensitivity. Figure 8-3 shows

the transimpedance circuit with the parameters defined in Section 8.2.1.1. To use the Excel calculator available

in What You Need To Know About Transimpedance Amplifiers – Part 1 to help with the component selection,

determine the total input capacitance, CTOT. In the calculator, CTOT is referred to as CIN. CTOT is the sum of CD,

CDIFF, and CCM, which is 7.4 pF. Using this value of CTOT, the targeted closed-loop bandwidth (f–3dB) of 24 MHz,

and the transimpedance gain of 100 kΩ results in the need for an amplifier with approximately 2.68 GHz GBWP

and a feedback capacitance (CF) of 0.092 pF. Table 8-2 shows the calculator results. 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.

Table 8-2. Results of Inputting Design Parameters in

the TIA Calculator

Calculator II

Closed-loop TIA bandwidth (f–3dB)

24.00

MHz

Feedback resistance (RF)

100

kΩ

Input capacitance (CIN)

7.40

pF

Op-amp gain bandwidth product (GBWP)

2678.14

MHz

Feedback capacitance (CF)

0.092

pF

OPA818

SBOS940B – MAY 2019 – REVISED DECEMBER 2025

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