Figure 1-1 illustrates the minimum requirement for an op amp model that can be used for stability testing. The model must accurately match the real A_OL over frequency, the open-loop output impedance, and input capacitance of the op-amp. The DC A_OL value is modeled with the voltage-controlled voltage source on the input (VCVS1 = –1M V/V, for 120dB). The dominant pole is modeled with simple RC low pass filters. The output of the low pass filter is buffered with a voltage-controlled voltage source in a gain of 1V/V. The open-loop output impedance (Z_O) is modeled with a simple resistor in this case, but many devices require a complex impedance.

Figure 1-3 illustrates the graph of A_OL for Figure 2-3. Notice that the DC A_OL corresponds to VCVS1, and the dominant pole corresponds to 1/(2 × π × R1 × C1). Note that the dominant pole imparts a –90° phase shift. The phase shift at low frequency is 180° due to the op amps negative feedback. Beyond the dominant pole, many op amps have multiple secondary poles and zeros near the unity gain frequency of the amplifier, as well as input capacitance (see Modeling the A_OL Secondary Poles and Zeros and Input Capacitance for details).

Figure 2-4 illustrates the open-loop output impedance for the model shown in Figure 2-3. This model uses a resistive open-loop output impedance (Z_O). Resistive Z_O (denoted as R_O) is the easiest to understand and also the better case from a stability perspective. The initial theory, assuming Z_O is resistive, is explained in this document, and later shows how a complex Z_O can impact stability. Practical op amps can have a resistive Z_O or complex Z_O. Modeling the output impedance of an op amp for stability analysis explains how to build a model with a complex Z_O. Most Texas Instruments' models accurately model both Z_O and A_OL over frequency so individually developing these models is generally unnecessary.