7.3.4 Output Impedance

The open-loop output impedance of the OPAx320-Q1 common-source output stage is approximately 90 Ω. When the op amp is connected with feedback, this value is reduced significantly by the loop gain. For example, with 130 dB (typical) of open-loop gain, the output impedance is reduced in unity-gain to less than 0.03 Ω. For each decade rise in the closed-loop gain, the loop gain is reduced by the same amount, which results in a ten-fold increase in effective output impedance. While the OPAx320-Q1 output impedance remains very flat over a wide frequency range, at higher frequencies the output impedance rises as the open-loop gain of the op amp drops. However, at these frequencies the output also becomes capacitive as a result of parasitic capacitance. This in turn prevents the output impedance from becoming too high, which can cause stability problems when driving large capacitive loads. As mentioned previously, the OPAx320-Q1 have excellent capacitive load drive capability for op amps with the bandwidth.