Op-Amp Models with ABM Parts

It is easy to create ideal parts using ABM parts. The circuit below has infinite input impedance and a gain of 

If we make a subcircuit out of this circuit, we could run into problems when the circuit is used. A problem that may result is that, if any of the terminals are not connected to other circuit elements, PSpice may generate an error message that only one element is connected to a certain node. To avoid this problem, we will add resistors to the circuit that do not affect its operation  

The resistors have a value of IT or 1012 2. These resistors are so large that they will not affect the operation of the circuit in most cases, and they avoid the problem of only one circuit element being connected to each node. 

The circuit below is an improved model for an op-amp. It has a low frequency gain of 

The output voltage swing is limited to 15 V and we have added a pole at 30 rad/sec. This is approximately the frequency response of a 741 op-amp. Note that this circuit is still ideal because it does not include many of the other nonideal characteristics of a 741 op-amp such as bias currents, offset voltages, and slew rate. 

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