INA821 datasheet(26/47 Pages) TI

26 / 47 page

9.2.1.3 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 9-4 shows how the INA597 closed-loop

output impedance varies over frequency.

Frequency (Hz)

0.0001

0.001

0.01

0.1

100

1000

100

10k

100k

10M

D073

G = 2

G = 0.5

VS = ±18 V

Figure 9-4. Closed-Loop Output Impedance vs Frequency

INA597

SBOS385B – AUGUST 2019 – REVISED APRIL 2021

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INA821 Datasheet(PDF) 26 Page - Texas Instruments