9.2.2 Noninverting Gain of 3 V/V

The OPAx607-Q1 devices are normally stable in noise gain configurations (see SBOA066) of greater than 6

V/V when conventional feedback networks are used, which is discussed in Section 8.3.4. The OPAx607-Q1

devices can be configured in noise gains of less than 6 V/V by using capacitors in the feedback path and

between the inputs to maintain the desired gain at lower frequencies and increase the gain greater that 6

V/V at higher frequencies such that the amplifier is stable. Configuration (a) in Figure 9-6 shows OPAx607-Q1

devices configured in a gain of 3 V/V by using capacitors and resistors to shape the noise gain and achieve a

phase margin of approximately 56° that is very close to the phase margin achieved for the conventional 6 V/V

configuration (b) in Figure 9-6.

The key benefit of using a decompensated amplifier (such as the OPAx607-Q1) below the minimum stable

gain, is that it takes advantage of the low noise and low distortion performance at quiescent powers smaller

than comparable unity-gain stable architectures. By reducing the 100-pF input capacitor, higher closed-loop

bandwidth can be achieved at the expense of increased peaking and reduced phase margin. Ensure that low

parasitic capacitance layout techniques on the IN– pin are as small as 1 pF to 2 pF of parasitic capacitance

on the inverting input, which will require tweaking the noise-shaping component values to get a flat frequency

response and the desired phase margin. Configurations in Figure 9-6 does not take into account this parasitic

capacitance but it must be considered for practical purposes. Details on the benefits of decompensated

architectures are discussed in Using a decompensated op amp for improved performance. The one-capacitor,

externally compensated type method is used for noise gain shaping in the below circuit.

In a difference amplifier circuit, typically used for low side current sensing applications, the (noise gain) = (signal

gain + 1).

+5 V

0 V

+

±

2 k

2.5 pF

OPAx607-Q1

1 k

100 pF

(a) G = 3 V/V

+5 V

0 V

+

±

OPAx607-Q1

1 k

(b) G = 6 V/V

470

5 k

2.5 pF

+

±

+5 V

1 NŸ

1NŸ

LOAD

GND

2 NŸ

2 NŸ

0V

2.5 pF

2.5 pF

GND

(c) G = 2 V/V

OPAx607-Q1

100 pF

470

Figure 9-6. Noninverting Gain of 3 V/V, 6 V/V Configurations, and Difference Amplifier in Signal Gain of 2

V/V

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SBOSA38A – FEBRUARY 2021 – REVISED APRIL 2021

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