SPICE newbie question

Hi,

I want to know how to vary a parameter inside a model and get the results in one run (one simulation). I've given a simple circuit below that I would also want to do .TRAN and .AC analysis on. (Like run an AC response for three transistor values of Bf.) I've managed to do .TEMP parameterization, but I can't figure out how to do something like Bf.

For the purpose of example, what syntax would I add to parameterize the Beta_forward (Bf) to values of 50, 125, and 300? (The model is the one that comes with LTSPICE, BTW.)

I have been trying to learn on the latest version LTSPICE, if that matters.

Thanks, Pete

Here is an example circuit:

---------------------------

2n3904_00

• Vcc

• .-----.

• | |

• ---V1 |

• - 5V .-.

• | | |R1

• === | |2k7

• GND '-'

• |

• .-----o

• | |

• .-. |Vc

• | |R3 |

• | |13k |

• '-' |

• |Vb |/ 2n3904

• o---| Q1

• | |>

• .-. |

• | |R2 |

• | |6k8 |

• '-' |

• | |

• '-----o

• |

• ===

• GND

* * *(created by AACircuit v1.28.6 beta 04/19/05

Q1 Vc Vb 0 0 2N3904 R1 Vcc Vc 2.7k R2 Vb 0 6.8k R3 Vc Vb 13k V1 Vcc 0 5 .model 2N3904 NPN (

+IS=1E-14 +VAF=100 +Bf=300 +IKF=0.4 +XTB=1.5 +BR=4 +CJC=4E-12 +CJE=8E-12 +RB=20 +RC=0.1 +RE=0.1 +TR=250E-9 +TF=350E-12 +ITF=1 +VTF=2 +XTF=3 +Vceo=40 +Icrating=200m +mfg=Philips +) .op .end