Spectrum Software
Industrial Strength Simulation




Spectrum Software has released Micro-Cap 11, the eleventh generation of our SPICE circuit simulator.

For users of previous Micro-Cap versions, check out the new features available in the latest version. For those of you who are new to Micro-Cap, take our features tour to see what Micro-Cap has to offer.




Questions and Answers - Spring 2003


Question: I am running transient analysis on my circuit. There is an initial transient start-up in the simulation that Iím not interested in. I would like to start the simulation at the steady state point. Is there a way to bypass the initial transient?

Answer: The initial transient would have to be simulated. However, once the transient is complete, you can save the data at that point of the simulation and use it to initialize another simulation run. The operation for that is as follows:

1) Set the time range in the Transient Analysis Limits to the time at which you want to save the data.
2) Run the simulation.
3) Go to the Transient menu and choose State Variables Editor.
4) Click on the Write command button and save the file name as circuitname.top where circuitname is the name of your circuit. Close the State Variables Editor.
5) Go to the Transient menu and choose Limits.
6) In the limits dialog box, disable the Operating Point option if it is on and change the State Variables option to Read.
7) Set the time range to the length you want to simulate and run the simulation.

Question: Iím using a 2N4265 NPN BJT in my circuit. The circuit seems to run fine, but I was looking at the model parameters for this BJT and the BF is specified at 1.13172K. That seems way too high in comparison to the data sheet values of hFE. What am I missing here?

Answer: In SPICE, many of the parameters in the model statement are curve fitting parameters. These often donít have a direct correlation to the data sheet values. For a BJT, the Beta vs Ic curve is actually calculated using four parameters: BF, NE, ISE, and IKF. The ISE and NE parameters model the additional base current that reduces the beta at low base-emitter voltages. The IKF parameter models the high-current reduction in beta.

To see the actual beta curve in Micro-Cap, use the Plot feature in the Attribute dialog box. Choose to plot the DC Current Gain and it will display the beta curve for that transistor model.

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