8.1.2 Inverting Amplifier

The OPAx837 can be used as an inverting amplifier with a signal input to the inverting input, V_IN–, through the gain-setting resistor R_G. A basic block diagram of the circuit is illustrated in Figure 63.

The output of the amplifier can be calculated according to Equation 3 if V_IN = V_REF + V_SIG and the noninverting input is biased to V_REF.

Equation 3.

The signal gain of the circuit is set by Equation 4 and V_REF provides a reference point around which the input and output signals swing. For bipolar-supply operation, V_REF is often ground. The output signal is 180˚ out-of-phase with the input signal in the pass band of the application. Figure 75 shows the 50-Ω input matched configuration used for the inverting characterization plots set up for a gain of –1 V/V. In this case, an added termination resistor, R_T, is placed in parallel with the input R_G resistor to provide an impedance match to 50-Ω test equipment. The output network appears as a 2-kΩ load but with a 50-Ω source to the network analyzer. This output interface network does add a 37.9-dB insertion loss that is normalized out in the characterization curves. Table 3 lists the suggested values for R_F, R_G, and R_T for inverting gains from –0.5 V/V to –10 V/V. If a 50-Ω input match is not required, eliminate the R_T element.

Equation 4.

Figure 75. Inverting Characterization Circuit for Network Analyzer