8.1.2 Selection of Feedback Resistors

The OPA810-Q1 is a classic voltage-feedback amplifier with each channel having two high-impedance inputs

and a low-impedance output. Standard application circuits (see also Figure 8-3 and Figure 8-4) include the

noninverting- and inverting-gain configurations. The dc operating point for each configuration is level-shifted

by the reference voltage VREF, which is typically set to midsupply in single-supply operation. VREF is often

connected to ground in split-supply applications.

+

–

VIN

RG

VREF

VREF

RF

VSIG

VREF

(1+RF/RG)VSIG

VO

VS+

VS–

Figure 8-3. Noninverting Amplifier

+

–

VIN

RG

VREF

VREF

RF

VSIG

VREF

–(RF/RG)VSIG

VO

VS+

VS–

Figure 8-4. Inverting Amplifier

Equation 3 shows the closed-loop gain of an amplifier in noninverting configuration.

VO = VIN1 + RFRG  + VREF

(3)

Equation 4 shows the closed-loop gain of an amplifier in an inverting configuration.

VO = VIN − RFRG  + VREF

(4)

The magnitude of the low-frequency gain is determined by the ratio of the magnitudes of the feedback resistor

(RF) and the gain setting resistor RG. The order of magnitudes of the individual values of RF and RG offer a

trade-off between amplifier stability, power dissipated in the feedback resistor network, and total output noise.

The feedback network increases the loading on the amplifier output. Using large values of the feedback resistors

reduces the power dissipated at the amplifier output. Conversely, large feedback-resistor values increase the

inherent voltage and amplifier current noise contribution seen at the output while lowering the frequency at

which a pole occurs in the feedback factor (β). This pole causes a decrease in the phase margin at zero-gain

crossover frequency and potential instability. Using small feedback resistors increases power dissipation and

also degrades amplifier linearity due to a heavier amplifier output load. Figure 8-5 illustrates a representative

schematic of the OPA810-Q1 in an inverting configuration with the input capacitors shown.

+

–

VIN

VREF

VSIG

VREF

–(RF / RG)VSIG

VO

VS+

VS–

RG

CPCB

RG

CDIFF

CCM

CCM

Figure 8-5. Inverting Amplifier With Input Capacitors

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OPA810-Q1

SLOSEE7 – MAY 2025

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