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I simulated circuit in Multisim and I found transistor model. I notice BF in spice model so I think "Is this the same with beta ?". If it's same why BF is constant ? When beta is not constant.

For example :

.MODEL 2N2222_Multisim npn +IS=1.87573e-15 BF=153.575
NF=0.897646 VAF=10

Heroz
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    Does this help? https://electronics.stackexchange.com/questions/590013/bjt-current-gain-relation-of-spice-parameter-bf-to-beta – GodJihyo Oct 25 '22 at 13:07
  • [The SPICE manual](http://bwrcs.eecs.berkeley.edu/Classes/IcBook/SPICE/UserGuide/elements.html#626) says that `BF` is the "ideal maximum forward beta". Key word there is "maximum". – Ste Kulov Oct 25 '22 at 21:08

1 Answers1

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BF is the parameter used to describe the maximum beta of the device. Effective beta changes based on current - at low currents, base-emitter leakage and non-ideal B-E junction effects reduce the effective beta; at high currents, other non-ideal effects (high level injection) also reduce effective beta.

While beta is useful To broadly describe BJT performance, it is not a fundamental parameter of the device.

Beta is not used directly in a transistor to make IC=beta*IB; rather IB is calculated from B-E characteristics (area, injection efficiency and recombination); then IC is calculated from similar parameters. The result (IC/IB) can be characterized as 'beta', but it is not the primary variable.

jp314
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    So far as I'm aware, BF is a parameter to the simulation model, not so much a 'maximum' or 'minimum' though I'm sure there's plenty of documentation suggesting the idea. Instead, the modified Gummel-Poon forward current gain of the BJT is defined by these model parameters: IS, BF, NF, ISE, IKF and NE. Note that BF is an *input* to the model and is directly applied in the formulas used during simulation. (The reverse current gain is defined by these parameters: IS, BR, NR, ISC, IKR and NC.) – jonk Oct 25 '22 at 19:38
  • But why it's constant? – Heroz Oct 25 '22 at 23:52
  • BF is a constant; beta (which depends on BF) is not constant. The actual BF value that is used in the equations also contains an exponential modification of BF using a parameter XTB. – jp314 Oct 26 '22 at 01:50
  • @Heroz It's just a starting point for simulation. It's set to a value that is appropriate at the default operating temperature (usually this is 300 Kelvin.) Things are then adjusted from there. – jonk Oct 26 '22 at 05:38