Ebers–Moll model aF? Where does it come from?

Question

I am just getting back into electronics after a long while. For the life of me I can't figure where the \$\alpha_F\$ variable in the Ebers–Moll model comes from when you are calculating \$I_C\$ or \$I_B\$ using the formulas given under the Ebers–Moll section on the BJT Wiki Page. My first instinct is that it is \$h_{fe}\$ , if I am to take the meaning of 'common base forward short-circuit current gain' literally, since \$h_{fe}\$ was the closest rating I could find in any datasheet.

So what I am asking is: Where do you get \$\alpha_F\$ when you are using the Ebers–Moll model of a BJT?

score 2 · Accepted Answer · answered Feb 15 '17 at 20:46

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αF is not the Hfe of the transistor. Per the model, αF of the emitter current reaches the collector.

This means that αF of the diode current passing through the base-emitter junction contributes to the current flowing through the base-collector junction.

Typically, αF has a value of between 0.98 and 0.99. the forward beta (~hfe) is αF/(1-αF).

answered Feb 15 '17 at 20:46

John D

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I did not think it was hfe! So what would I put into the equation then? – Zack Frost Feb 15 '17 at 20:48
Use hfe on the datasheet and the relation in the answer to estimate αF: aF = hfe/(1+hfe). – John D Feb 15 '17 at 20:52
Ok! That makes sense now. I missed that part at the end for whatever reason. – Zack Frost Feb 15 '17 at 20:56

score 0 · Answer 2 · answered Feb 15 '17 at 20:45

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It is defined as function of collector and emitter current. More specifically a ratio of the exponential functions which describe the current at the emitter and collector. You should find that the forward ratio is near unity.

answered Feb 15 '17 at 20:45

Samee87

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Ebers–Moll model aF? Where does it come from?

2 Answers2