Operating the OPA698 as an inverting amplifier has several benefits and is particularly useful when a matched 50Ω source and input impedance are required. Figure 8-10 shows the inverting gain of –2 circuit used as the basis of the inverting mode typical characteristics.

In the inverting case, only the feedback resistor appears as part of the total output load in parallel with the actual load. Using the 500Ω load used in the typical characteristics gives a total load of 222Ω in this inverting configuration. The gain resistor is set to get the desired gain (in this case, 200Ω for a gain of –2V/V) while an additional input resistor (R_M) can be used to set the total input impedance equal to the source, if desired. In this case, R_M = 66.5Ω in parallel with the 200Ω gain setting resistor gives a matched input impedance of 50Ω. This matching is only needed when the input needs to be matched to a source impedance, as in the characterization testing done using the circuit of Figure 8-10.

For bias current-cancellation matching, the noninverting input requires a 147Ω resistor to ground. The calculation for this resistor includes a dc-coupled 50Ω source impedance along with R_G and R_M. Although this resistor provides cancellation for the bias current, the circuit must be well-decoupled (0.1µF in Figure 8-10) to filter the noise contribution of the resistor and the input current noise.

As the required R_G resistor approaches 50Ω at higher gains, the bandwidth for the circuit in Figure 8-10 exceeds the bandwidth at that same gain magnitude for the noninverting circuit of Figure 8-8. This result occurs because of the lower noise gain for the circuit of Figure 8-10 when the 50Ω source impedance is included in the analysis. For instance, at a signal gain of –10V/V (R_G = 50Ω, R_M = open, R_F = 500Ω) the noise gain for the circuit of Figure 8-10 is 1 + 500Ω/(50Ω + 50Ω) = 6 due to the addition of the 50Ω source in the noise gain equation. This approach gives considerably higher bandwidth than the noninverting gain of 10V/V. Using the 250MHz gain bandwidth product for the OPA698, an inverting gain of –10V/V from a 50Ω source to a 50Ω R_G gives 52MHz bandwidth; whereas, the noninverting gain of 10V/V gives 28MHz, as shown in Figure 8-11.