8 Detailed Description

8.1 Overview

The ultra-wide, 8-GHz gain bandwidth product (GBWP) of the OPA855, combined with the broadband voltage

noise of 0.98 nV/√Hz, produces a viable amplifier for wideband transimpedance applications, high-speed data

acquisition systems, and applications with weak signal inputs that require low-noise and high-gain front ends.

The OPA855 combines multiple features to optimize dynamic performance. In addition to the wide, small-signal

bandwidth, the OPA855 has 850 MHz of large-signal bandwidth (2 VPP), and a slew rate of 2750 V/µs, making

the device a viable option for high-speed pulsed applications.

8.2 Functional Block Diagram

The OPA855 is a classic voltage-feedback operational amplifier (op amp) with two high-impedance inputs and

a low-impedance output. Standard application circuits are supported, such as the two basic options in Figure

8-1 and Figure 8-2. The resistor on the noninverting pin is used for bias current cancellation to minimize the

output offset voltage. In a noninverting configuration, the additional resistors on the noninverting pin add noise

to the system; therefore, if SNR is critical, the resistor can be eliminated. In an inverting configuration, the

noninverting node is typically connected to a dc voltage; therefore, the high-frequency noise contribution from

the bias cancellation resistor can be bypassed by adding a large 1-µF capacitor in parallel to the resistor to

shunt the noise. The dc operating point for each configuration is level-shifted by the reference voltage (VREF),

which is typically set to midsupply in single-supply operation. VREF is typically connected to ground in split-supply

applications.

+

±

VIN

RG

VREF

VREF

RF

VSIG

VREF

(1+RF/RG)×VSIG

VOUT

VS+

VS±

RF || RG

Figure 8-1. Noninverting Amplifier

+

±

VIN

RG

VREF

VREF

RF

VSIG

VREF

±(RF/RG)×VSIG

VOUT

VS+

VS±

RF || RG

Figure 8-2. Inverting Amplifier

OPA855

SBOS622D – JULY 2018 – REVISED MAY 2025

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