8 Application and Implementation

Note

Information in the following applications sections is not part of the TI component specification,

and TI does not warrant its accuracy or completeness. TI’s customers are responsible for

determining suitability of components for their purposes, as well as validating and testing their design

implementation to confirm system functionality.

8.1 Application Information

8.1.1 Wideband, Noninverting Operation

The OPA818 provides a unique combination of high GBWP, low-input voltage noise, and the dc precision of

a trimmed JFET-input stage to provide an exceptionally high input impedance for a voltage-feedback amplifier.

The high GBWP of 2.7 GHz is used to either deliver high-signal bandwidths at high gains, or to extend the

achievable bandwidth or gain in photodiode-transimpedance applications. To achieve the full performance of

the OPA818, pay careful attention to printed circuit board (PCB) layout and component selection; see also the

following sections of this data sheet.

Figure 8-1 shows the noninverting gain of +7 V/V circuit used as the basis for most of the Typical Characteristics:

VS = ±5 V. Most of the curves are characterized using signal sources with 50-Ω driving impedance, and with

measurement equipment presenting a 50-Ω load impedance. In Figure 8-1, the 49.9-Ω shunt resistor at the

VIN terminal matches the source impedance of the test generator, while the 49.9-Ω series resistor at the VO

terminal provides a matching resistor for the measurement equipment load. Generally, data-sheet voltage-swing

specifications are at the output pin (VO in Figure 8-1) while output power specifications are at the matched 50‑Ω

load. The total 100-Ω load at the output combined with the 350-Ω total feedback network load, presents the

OPA818 with an effective output load of 78 Ω for the circuit of Figure 8-1.

R

F

301

–5 V

+5 V

R

G

49.9

0.01

F

0.22

F

0.01

F

0.22

F

49.9

V

IN

V

O

49.9

+

–

50

Source

50

Load

Figure 8-1. Noninverting G = +7 V/V Configuration and Test Circuit

Voltage-feedback operational amplifiers, unlike current feedback products, use a wide range of resistor values

to set gain. To retain a controlled frequency response for the noninverting voltage amplifier of Figure 8-1,

ensure that the parallel combination of RF || RG is less than 50 Ω. In the noninverting configuration, the parallel

combination of RF || RG forms a pole with the parasitic input capacitance at the inverting node of the OPA818

(including layout parasitics). For best performance, set this pole to a frequency greater than the closed-loop

bandwidth for the OPA818.

OPA818

SBOS940B – MAY 2019 – REVISED DECEMBER 2025

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