2 Circuit Configuration Impact on Common-Mode Range

The Introduction showed how the common-mode limitation of the amplifier can be calculated given the supply voltages and the data sheet specification table. The amplifier circuit configuration often determines if this limitation causes nonlinear operation. The voltage follower is the configuration that is most likely to be impacted by common-mode limitations. This is because the common-mode voltage is the same as the input signal and the input signal often covers the entire supply range. Figure 2-1 provides a DC input sweep to illustrate the common-mode limitation of the OPA206 in a buffer configuration. Note that the input voltage is equal to the common-mode signal in this configuration, so the linear range is –4 V to 10.6 V as calculated in Figure 1-1.

Figure 2-1 Common-Mode Limit of OPA206 in Buffer Configuration

The same op amp in a gain configuration is not impacted by the common-mode limitations because the valid input signal range for the gain configuration is small and does not approach the common-mode limits. For example, in a gain of 10 V/V, the input range is ten times smaller than the output range. Figure 2-2 shows the same supply arrangement in a gain of 10 V/V. In this case, the best output range is –5 V to +12 V, so the valid input range is –0.5 V to 1.2 V. Since the common-mode range for this device is –4.0 V to +10.6 V, the valid input signal is never near the common-mode limits. For this example, notice that the output signal does not swing all the way to the supply limits. This limitation is because of the output swing limitations, and not the common-mode limits and are covered in detail later in this white paper.

Figure 2-2 Common Mode Does not Limit OPA206 in Non-inverting Gain of 10 V/V

In the non-inverting configuration, the common-mode voltage is equal to the input signal. For the inverting configuration the common-mode voltage is equal to the voltage applied to the non-inverting input. In general, for inverting amplifiers the non-inverting input is connected to ground or a fixed DC voltage. For inverting amplifiers, the common-mode voltage remains constant regardless of the input signal, so these types of amplifiers generally do not have common-mode issues. The example shown in Figure 2-3 shows that the common-mode signal is held constant at ground, and the amplifier does not experience common-mode limitations. This amplifier has a gain of –1 V/V, so you can substitute this configuration for a buffer configuration if common-mode limitation is a problem for the buffer. However, remember that the inverting amplifier configuration has gain error and drift due to the tolerance of the feedback resistors, whereas the buffer has a very accurate gain of 1 V/V. Furthermore, the resistors add noise and additional power consumption for the amplifier.

Figure 2-3 Common Mode Does not Limit OPA206 in Inverting Gain Configuration