The OPA855IDSGR is a wideband, low-noise operational amplifier engineered specifically for transimpedance and voltage amplifier applications where the signal of interest is fast, weak, and originates from a low-capacitance photodiode. Unlike general-purpose high-speed amplifiers that must compromise between bandwidth, noise, and input bias current, the OPA855IDSGR is a decompensated bipolar-input device that prioritizes raw gain-bandwidth and noise performance for optical front-end designs. Housed in an 8-pin WSON package and rated for operation from –40°C to 125°C, it serves as the first active stage in LIDAR receivers, optical time-of-flight ranging systems, and high-resolution laser distance measurement equipment.

The defining specification of the OPA855IDSGR is its 8 GHz gain-bandwidth product. This extremely high GBWP is achieved through a decompensated architecture that is stable only for closed-loop gains of 7 V/V or greater. This design choice is deliberate: transimpedance amplifiers operate at noise gains determined by the feedback network and photodiode capacitance, which are typically well above unity. By optimizing for high gain configurations, the device delivers bandwidth that a unity-gain-stable amplifier of comparable power could not achieve.

The input stage uses bipolar transistors, which yields an input voltage noise of 0.98 nV/√Hz. This low voltage noise is essential for resolving small photocurrent pulses against the thermal noise of the feedback resistor. The slew rate of 2750 V/µs ensures that the amplifier can respond to nanosecond-scale optical pulses without slew-limiting, while the 2.5 GHz –3 dB bandwidth supports closed-loop transimpedance bandwidths in the hundreds of megahertz at practical transimpedance gains.

A particularly important parameter for photodiode interfacing is the input capacitance. The common-mode input capacitance of 0.6 pF and differential input capacitance of 0.2 pF minimize the total capacitance at the amplifier input node. In a transimpedance configuration, the total input capacitance—comprising the photodiode capacitance plus the amplifier input capacitance—sets the feedback pole that must be compensated to maintain stability. Lower amplifier input capacitance allows higher transimpedance gain and greater bandwidth for a given photodiode.

The OPA855IDSGR incorporates a dedicated feedback pin that simplifies the connection of the feedback network between the amplifier input and output. This pin placement is not merely a convenience; it minimizes parasitic inductance and capacitance in the feedback path, which is critical for stability at gigahertz frequencies. The wide input common-mode range extends to within 1.4 V of the positive supply and includes the negative supply, accommodating both single-supply and split-supply operation.

The device includes a power-down mode that reduces quiescent current when the amplifier is not actively acquiring data. This feature is particularly valuable in battery-powered LIDAR systems or portable optical instruments where duty-cycled operation extends battery life. The output stage can deliver 105 mA, sufficient to drive the input of a high-speed analog-to-digital converter either directly or through a differential output amplifier.

The primary application of the OPA855IDSGR is the optical front-end of time-of-flight and LIDAR systems. In these systems, a short pulse of light is emitted and the returning reflection is focused onto a photodiode. The photodiode generates a current pulse proportional to the received optical power, and the transimpedance amplifier converts this current into a voltage for subsequent time-to-digital conversion or analog-to-digital conversion. The 8 GHz GBWP and 0.98 nV/√Hz noise floor allow the detection of weak return pulses at distances where the optical signal is attenuated by free-space propagation.

Beyond LIDAR, the device is applicable to optical time-domain reflectometry for fiber optic fault location, laser distance measurement for industrial metrology, 3D scanners for machine vision, and solid-state scanning LIDAR for autonomous navigation. In drone vision and industrial robot LIDAR systems, the compact WSON package and wide temperature range support integration into space-constrained and thermally demanding environments.

The OPA855IDSGR represents a focused engineering trade-off: unity-gain stability is sacrificed in exchange for bandwidth and noise performance that are optimized for transimpedance operation. This trade-off is well matched to optical sensing applications, where the amplifier operates at noise gains well above unity and the limiting factors are the photodiode capacitance and the feedback network. For designers of high-resolution optical receivers, the device provides a first-stage amplifier that preserves signal bandwidth and fidelity from the photodiode to the digitizer.