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Why Do Some Opamps Output Extremely High Voltages?

Primary Software: Electronics Workbench>>Multisim
Primary Software Version: 8.2.5
Primary Software Fixed Version: N/A
Secondary Software: N/A

Problem:

Despite connecting an opamp with a five-pin schematic model to the appropriate power supply voltages (voltage rails), the opamp does not reach saturation during simulation, or in some cases it exceeds the voltage output rating based on the supplies.

Why is this?



Solution:

Certain components in Multisim database, such as opamps, have multiple SPICE models associated with a single symbol. So in choosing a number of opamps for simulation in Multisim, an additional step of selecting the model is also necessary.

Generally, even though the symbol for an opamp component has 5 pins, the actual SPICE model associated with it may be either 3 or 5 pins. If a 3-pin SPICE model is selected, connecting the power supply voltages to the schematic symbol will have no effect on the simulation output and therefore an opamp will not reach saturation, or it will exceed the voltage output rating.

When looking at the actual SPICE model of a component such as an opamp, the first line indicates the number of pins that will be simulated in the model. If the first line looks like:

.Subckt LM741 1 2 3

... you are looking at a 3-pin model (as denoted by the 3 pins - 1 2 3 - listed after the model name). If instead the first line looks like:

.Subckt LM741 1 2 3 90 45

... you are looking at a 5-pin model (as denoted by the 5 pins - 1 2 3 90 45 - listed after the model name).

In order to ensure that a 5-pin SPICE model has been selected you must check the model when placing the component onto the schematic. To do this:

  1. Select Place » Component to open up the component placement tool
  2. Navigate to the Opamp family in the Analog group of the Master Database
  3. Select an Opamp model in the component column (such as the LM324N)
  4. Select a SPICE model for this component in the Model Manuf.\ID area (circled in red in the image below)
  5. To view the actual SPICE model now click on the Model button (circled in red in the image below).

You can now ensure that you have selected a 3- or 5-pin SPICE model for an Opamp before placing it onto the schematic.

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Report Date: 07/28/2006
Last Updated: 06/10/2008
Document ID: 3ZRD9GDK

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