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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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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