INA821 datasheet(27/45 Pages) TI

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8.2.1.3 Application Curve

The interaction between the output stage of an operational amplifier (op amp) and capacitive loads can impact

the stability of the circuit. Throughout the industry, op-amp output-stage requirements have changed greatly

since their original creation. Classic output stages with the class-AB, common-emitter, bipolar-junction transistor

(BJT) have now been replaced with common-collector BJT and common-drain, complementary metal-oxide

semiconductor (CMOS) devices. Both of these technologies enable rail-to-rail output voltages for single-supply

and battery-powered applications. A result of changing these output-stage structures is that the op-amp open-

loop output impedance (ZO) changed from the largely resistive behavior of early BJT op amps to a frequency-

dependent ZO that features capacitive, resistive, and inductive portions. Proper understanding of ZO over

frequency, and also the resulting closed-loop output impedance over frequency, is crucial for the understanding

of loop-gain, bandwidth, and stability analysis. Figure 8-4 shows how the INA592 closed-loop output impedance

varies over frequency.

Frequency (Hz)

0.0001

0.001

0.01

0.1

100

1000

100

10k

100k

10M

G = 2

G = 0.5

VS = ±18 V

Figure 8-4. Closed-Loop Output Impedance vs Frequency

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INA592

SBOS914E – OCTOBER 2018 – REVISED FEBRUARY 2021

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INA821 Datasheet(PDF) 27 Page - Texas Instruments