OPA863 datasheet(23/52 Pages) TI

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8.3 Feature Description

8.3.1 Input Stage

The OPAx863 devices 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 till 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 will cause a crossover distortion which needs to be considered in high-frequency applications

requiring superior linearity. Limit the common-mode input voltage to VS+ – 1.6 V (maximum) for main-stage

operation across process and ambient temperature.

Since the OPAx863 devices are bipolar amplifiers, the two inputs are protected with anti-parallel back-to-back

diodes between them, 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 anti-parallel diodes begin to conduct in very fast input or

output transient conditions. Care must be taken to use gain-setting and feedback resistors large enough to limit

the current through these diodes in such conditions.

8.3.2 Output Stage

The OPAx863 devices 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 to either of the supplies will cause output signal clipping and introduce distortion.

The OPAx863 devices integrate an output short-circuit protection circuit, which makes the device rugged for use

in real-world applications.

8.3.2.1 Overload Power Limit

The OPAx863 devices include overload power limiting which limits the increase in device quiescent current with

output saturated to either of the supplies. Typically, when an amplifier's output saturates, its two inputs are pulled

apart which may enable the slew boost circuit. The input differential voltage is an error voltage in negative

feedback, which the amplifier core nullifies by engaging the slew boost circuit and driving the output stage

deeper into saturation. Once the input to an amplifier attains a value large enough to saturate its output, any

further increase in this input excitation results in a finite input differential voltage. As the output stage transistor

is pushed deeper into saturation, its hFE (base-to-collector current gain) drops with increase in its base and

collector current, increasing the device quiescent current. This may cause a catastrophic failure in multi-channel,

high-gain, high-density front-end designs and reduce operating lifetime in portable battery powered systems.

The OPAx863 devices overload power limiting includes an intelligent output saturation detection circuit which

limits the device's quiescent current to 2.2-mA per channel under DC overload conditions. This increase in

quiescent current is smaller with AC input or output and output saturation duration for only a fraction of the

overall signal time period. Table 8-1 compares the increase in quiescent current with 50 mV input overdrive for

OPAx863 devices and other voltage feedback amplifiers without overload power limit.

Table 8-1. Quiescent Current with Saturated Outputs

Device

Input Differential Voltage

Quiescent Current during

overload

Increase in IQ from

steady-state condition

OPAx863 with overload power limit

50 mV

1.1 mA

1.57x

Competitor amplifier without overload power limit

50 mV

1.96 mA

3.43x

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OPA863, OPA2863, OPA4863

SBOS982I – JUNE 2020 – REVISED DECEMBER 2022

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