An op-amp will in general have a small resistive output impedance from the push-pull output stage. We will model the open-loop output impedance by adding a series resistor to the output of an ideal op-amp as shown in figure 6.28.
Figure 6.28: Real, current-limiting operational amplifier partially modeled by an ideal amplifier and an output resistor.
Assuming no current into the input terminals (unloaded), and hence no current through , we have . Using the open-loop transfer function we obtain
Shorting a wire across the output gives and hence
Using the standard definition for the impedance gives
If than , which is small as required by our infinite open-loop gain approximation.
We can now draw the impedance outside the feedback loop and use
The circuit can now be modeled by a resistor in series with an inductor all in parallel with another resistor (three passive components) as shown in figure 6.29. Students should convince themselves of this.
Figure 6.29: An equivalent circuit for a 741-type operational amplifier.
If the op-amp is used to drive a capacitive load, the inductive component in the output impedance could set up an LCR resonant circuit which would result in a slight peaking of the transfer function near the corner frequency as shown in figure 6.30
Figure 6.30: The overall transfer function when the amplifier drives a capacitive load.