The zero in Equation 2 increases the noise gain at higher frequencies, which is important when compensating

a decompensated amplifier. However, the noise gain zero reduces the loop gain phase which results in a lower

phase margin. To counteract the phase reduction due to the noise gain zero, add a pole to the noise gain curve

by inserting the 0.5-pF feedback capacitor. The pole occurs at a frequency shown in Equation 3. The noise

gain pole and zero locations must be selected so that the rate-of-closure between the magnitude curves of AOL

and 1/β is approximately 20 dB. To provide this rate of closure, the noise gain pole must occur before the 1/β

magnitude curve intersects the AOL magnitude curve. In other words, the noise gain pole must occur before |AOL|

= |1/β|. The point at which the two curves intersect is known as the loop gain crossover frequency.

f= 12πRFCF

(3)

where

• CF is the feedback capacitor (includes any added PCB parasitic)

For more information on op-amp stability, watch the TI Precision Lab series on stability video.

9.2.1.3 Application Curves

Frequency (Hz)

-16

-12

-8

-4

0

4

8

12

16

1M

10M

100M

1G

D206

Simulated Response

Measured Response

VOUT = 100 mVPP

Figure 9-6. Gain = –2 V/V, Uncompensated Frequency Response

Frequency (Hz)

-32

-28

-24

-20

-16

-12

-8

-4

0

4

1M

10M

100M

1G

D207

Simulated Response

Measured Response

VOUT = 100 mVPP

Figure 9-7. Gain = –2 V/V, Compensated Frequency Response

OPA858

SBOS629B – APRIL 2018 – REVISED MAY 2025

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