1 Introduction: Open-Loop Output Impedance (R_O)

The ideal op amp has infinite input impedance and zero output impedance. In practice, every real amplifier has a non-zero output impedance typically in the range of Ω to kΩ. Texas Instruments op amp data sheets include a plot of the open-loop output impedance (R_O) over frequency. R_O is not to be confused with the closed-loop output impedance (R_OUT), although the two are closely related by the loop gain.

Figure 1-1 shows the open-loop output impedance over frequency for the OPA392, which is flat at approximately 120 Ω across frequency. This flat portion of the curve implies that the open-loop output impedance acts as a resistor (R_O) across the effective bandwidth of the amplifier. Some amplifiers have an output impedance that changes over frequency, implying a complex impedance (Z_O). The analysis in this document applies to amplifiers that have a flat, resistive output impedance across the effective bandwidth of the amplifier.

Figure 1-1 Open-Loop Output Impedance vs Frequency, OPA392

R_O is also specified in the electrical characteristics section of the data sheet, which confirms the resistance is 120 Ω. This data sheet specification for R_O is used to determine an optimal value for R_ISO when driving a large capacitive load, as described in Section 3.