Monday, 16 June 2014

Using the .step command to step a parameter of a transistor model in LTspice



I read this document and I want to step the β parameter of a transistor for the below circuit(to see it bigger please left-click on the image below):


enter image description here


Here is the model of the transistor and BF is the β:



.model MyBJT NPN(IS=1E-14 VAF=100 BF=100 IKF=0.3 XTB=1.5 BR=3 CJC=8E-12 CJE=25E-12 TR=100E-9 TF=400E-12 ITF=1 VTF=2 XTF=3 RB=10 RC=.3 RE=.2 Vceo=30 Icrating=800m mfg=NXP)



Then in the above line I change BF to {BF}


And step BF as follows:



.step param BF 10 500 50




and then



.tran 1



But I'm not getting many plots just one.


Am I doing something wrong? Is it even possible what I'm trying to do? Is there a way?



Answer



Jramsay42 is correct.


See the help to .STEP:




.STEP -- Parameter Sweeps This command causes an analysis to be repeatedly performed while stepping the temperature, a model parameter, a global parameter, or an independent source. Steps may be linear, logarithmic, or specified as a list of values.


Example: .step oct v1 1 20 5


Step independent voltage source V1 from 1 to 20 logarithmically with 5 points per octave.


Example: .step I1 10u 100u 10u


Step independent current source I1 from 10u to 100u in step increments of 10u.


Example: .step param RLOAD LIST 5 10 15


Perform the simulation three times with global parameter Rload being 5, 10 and 15.


Example: .step NPN 2N2222(VAF) 50 100 25


Step NPN model parameter VAF from 50 to 100 in steps of 25.



Example: .step temp -55 125 10


Step the temperature from -55°C to 125°C in 10-degree step. Step sweeps may be nested up to three levels deep.



Which works as you expect it. You also don't have to replace the value of your model by {BF} in that case, it will just overwrite the value of the model.


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