7.3 Feature Description

7.3.1 Input Stage

The OPAx863A include a rail-to-rail input stage. The main stage differential pair using PNP bipolar transistors

operates for common-mode input voltages from VS– – 0.2 V to VS+ – 1.6 V. The amplifier inputs transition into

the auxiliary stage using NPN transistors for common-mode input voltages from VS+ – 1.6 V till VS+ + 0.2 V. The

PNP and NPN input stages offer a gain-bandwidth product of 50 MHz and a voltage noise density of 6.3 nV/√Hz.

The offset voltage for the two input stages is matched to lie within the device specifications. The auxiliary NPN

input stage does not use the slew-boost circuit during large-signal transient response. The input bias current for

the PNP and NPN input stages is opposite in polarity, which adds an additional offset based on the values of

the gain-setting and feedback resistors. A common-mode input voltage transition between these input stages

causes a crossover distortion that must be considered in high-frequency applications requiring excellent linearity.

Limit the common-mode input voltage to VS+ – 1.6 V (maximum) for main-stage operation across process and

ambient temperature.

The OPAx863A are bipolar amplifiers; therefore, the two inputs are protected with antiparallel back-to-back

diodes between the inputs, which limits the maximum input differential voltage to 1 V. The amplifier is slew

limited, and the two inputs are pulled apart up to 1 V when the antiparallel diodes begin to conduct in very fast

input or output transient conditions. Make sure to use gain-setting and feedback resistors large enough to limit

the current through these diodes in such conditions.

7.3.2 Output Stage

The OPAx863A feature a rail-to-rail output stage with possible signal swing from VS– + 0.2 V to VS+ – 0.2 V.

Violating the output headroom of either supply causes output signal clipping and introduces distortion.

The OPAx863A integrate an output short-circuit protection circuit that makes the device rugged for use in

real-world applications.

7.3.2.1 Overload Power Limit

During overload or fault conditions, bipolar rail-to-rail output (RRO) amplifiers consume excessive quiescent

current (five to seven times) with saturated outputs. With saturated outputs, the output signal is clipped with

much higher base current from output predriver stage which results in increase in device quiescent current.

During this condition, the negative feedback control is disabled and an input differential voltage appears thereby

resulting in an input overdrive. During input overdrive, the slew boost circuit engages causing increase in the tail

current and hence the device quiescent current. This overall increase in quiescent current can cause excessive

battery discharge in portable products shortening operating lifetime or disturb the thermal equilibrium causing

irreversible damage due to increased system power dissipation in a multichannel design.

The OPAx863A includes an intelligent overload detection circuit that monitors for output saturation and limits the

base drive from output predriver circuit and disables the slew boost circuit in this condition. Table 7-1 compares

the increase in quiescent current with 500-mV input overdrive for OPAx863A devices and other voltage-feedback

amplifiers without overload power limit.

Table 7-1. Quiescent Current with Saturated Outputs

DEVICE

INPUT DIFFERENTIAL

VOLTAGE

QUIESCENT CURRENT

DURING OVERLOAD

INCREASE IN IQ

FROM STEADY-STATE

CONDITION

OPAx863A with overload power limit

500 mV

1.4 mA

1.8 ×

Competitor amplifier without overload power limit

500 mV

4.05 mA

7.1 ×

OPA863A, OPA2863A

SBOSA95D – MAY 2022 – REVISED DECEMBER 2023

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