8.2.2 Noninverting Gain of 2 V/V

The OPA818 is typically stable in noise-gain configurations greater than 7 V/V when conventional feedback

networks are used. The OPA818 is configurable in noise gains less than 7 V/V by using capacitors in the

feedback path and between the inputs. This configuration maintains the desired gain at lower frequencies and

increases the noise gain at higher frequencies such that the amplifier is stable. Configuration (a) in Figure

8-6 shows OPA818 configured in a gain of 2 V/V by using capacitors and resistors to shape the noise gain

and achieve a phase margin of approximately 51° that is very close to the phase margin achieved for the

conventional 7 V/V configuration (b) in Figure 8-6.

The key benefit of using a decompensated amplifier, such as the OPA818, at gains below the minimum stable

gain is that designers can take advantage of the low noise and low distortion performance at power levels

lower than those of comparable unity-gain stable architectures. The small-signal frequency response in Figure

8-6 shows flat ac performance beyond 100 MHz for a gain of 2 V/V configuration (a) in Figure 8-8, and by

being in a lower gain configuration versus the minimum stable gain configuration of 7 V/V, the output-referred

total noise is also lower (64 nV/√Hz at 100 MHz); see also Figure 8-8 compared to that at 166 nV/√Hz of

configuration (b). Reducing the 10-pF input capacitor allows the system to achieve higher closed-loop bandwidth

at the expense of increased peaking and reduced phase margin. Low-capacitance layout by minimizing trace

lengths and removing planes under the traces and components connected to the inverting input is critical to

minimize parasitic capacitance (see Layout Guidelines). Parasitic capacitance as small as 1 pF to 2 pF on the

inverting input requires tweaking the noise-shaping component values to get flat frequency response and the

desired phase margin. Configurations in Figure 8-6 do not take into account this parasitic capacitance but are

considered for practical purposes. A 45° phase margin is generally acceptable, but anything less than 40° is not

recommended to allow for component, PCB, and process tolerances. Details on the benefits of decompensated

architectures are discussed in Using a decompensated op amp for improved performance.

VO

+5 V

–5 V

+

–

301

3.3 pF

OPA818

VIN

301

10 pF

100

(a) G = 2 V/V

VO

+5 V

–5 V

+

–

301

OPA818

VIN

49.9

100

(b) G = 7 V/V

10

Figure 8-6. Noninverting Gain of 2 V/V and 7 V/V Configurations

Frequency (Hz)

-9

-6

-3

0

3

10M

100M

1G

D107

G = 2 V/V

G = 7 V/V

Figure 8-7. Small-Signal Frequency Response in

Gains of 2 V/V and 7 V/V Configurations of Figure

8-6

Frequency (Hz)

1

1

10

10

100

100

1000

1000

1M

10M

100M

D108

G = 2 V/V, eO

G = 7 V/V, eO

G = 2 V/V, EO

G = 7 V/V, EO

Figure 8-8. Output Noise in Gains of 2 V/V and

7 V/V Configurations of Figure 8-6

OPA818

SBOS940B – MAY 2019 – REVISED DECEMBER 2025

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