8.1.1 Noninverting Amplifier

The OPAx837 can be used as a noninverting amplifier with a signal input to the noninverting input, V_IN+. A basic block diagram of the circuit is illustrated in Figure 62. V_REF is often ground when split supplies are used.

Calculate the amplifier output according to Equation 1 if V_IN = V_REF + V_SIG.

Equation 1.

The signal gain of the circuit is set by Equation 2, and V_REF provides a reference around which the input and output signals swing. Output signals are in-phase with the input signals within the flat portion of the frequency response. For a high-speed, low-noise device such as the OPAx837, the values selected for R_F (and R_G for the desired gain) can strongly influence the operation of the circuit. For the characteristic curves, the noninverting circuit of Figure 74 shows the test configuration set for a gain of 2 V/V. Table 2 lists the recommended resistor values over gain.

Equation 2.

Figure 74. Characterization Test Circuit for Network, Spectrum Analyzer

Table 2 lists the recommended resistor values from target gains of 1 V/V to 10 V/V where standard E96 values are shown. This table controls the R_F and R_G values to set the resistor noise contribution at approximately 50% of the total output noise power. These values increase the spot noise at the output over what the op amp voltage noise produces by 41%. Lower values reduce the output noise of any design at the cost of more power in the feedback circuit. Using the TINA model and simulation tool shows the impact of different resistor value choices on response shape and noise.