# How to Create an Exponential Source SPICE Model

May 25, 2022

Modeling designs helps engineers verify whether their circuit will function as intended. Modeling programs simulate circuit behavior under different conditions and in accordance with the design requirements, allowing engineers to better plan and build circuits. Every SPICE simulation needs to have a source or input signal. To analyze how the circuit will behave in real life, this input must be modeled accurately. If not modeled accurately, functionality issues can go undetected until far later in the design process, wasting time and money. To efficiently and accurately model an exponential source, the intended input signal must be interpreted by identifying the type and signal values for replication.

What is a Exponential Source?

An exponential source is an exponentially rising and falling voltage or current source. The output voltage is an exponential pulse as a function of time.

What is needed to Model a Exponential Source?

To create the required exponential source for simulation, there are two items that must be defined:

● Type

○ Which type of signal will be the input for the circuit: voltage or current?

● Data Points

○ What are the critical signal values to create the desired waveform?

This information must be incorporated into the SPICE simulation model which can be achieved by manually creating or editing a text file. Keep in mind if the source signal created is not the intended outcome, values will need to be edited manually. This manual process to produce the desired source signal is time consuming and increases the likelihood of errors; however, the PSpice Modeling App provides a fast, easily-configurable, and fully-integrated method to create exponential sources for simulation.

PSpice Modeling App

The exponential source modeling application quickly creates exponential source models with a wizard-based approach. The necessary source specifications are pre-defined and users can easily input desired parameters required for all exponential sources such as:

● Type:

Select either a Voltage or Current

● Voltage or Current Levels:

Quickly define the initial and peak values for voltage or current.

● Rise/Fall Time:

Quickly specify the time it takes for the signal to increase from the initial value to the peak value and the time it takes for the signal to decrease from peak value to initial value.

● Rise/Fall Delay:

Efficiently define the time to delay the signal rise from the initial value to the peak value and the time to delay the signal fall from the peak value to the initial value.

● AC/DC:

Quickly specify the AC or DC voltage amplitude to be automatically added to source, achieving the intended AC sweep, bias point, or transient analysis.

Using the inputted information above, the PSpice Modeling App generates a schematic symbol and automatically associates the newly created exponential source SPICE model without leaving the OrCAD Capture environment. The PSpice Modeling App also automatically manages the simulation profile configuration, eliminating any library set up for simulation. To try this yourself, be sure to download the Free Trial of OrCAD and check back for more SPICE model how-tos in this series.

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