Question: I am running transient analysis on my circuit. There is
an initial transient start-up in the simulation that I’m 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: I’m 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 don’t 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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