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viren

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Pspice BJT Modeling
Jul 28^th, 2005, 8:48pm

I am trying to fit the DC characteristics of the real BJT which I had attached in my earlier email using a pspice BJT model. I am using the gummel poon model parameters to model the real BJT . I don't know which parameters to change . Since there is drop of gain at low currents I have to figure out how to model it in pspice model of BJt the same loss of gain BF . Since the equation for IB in the book had Ib (nonideal)= C2 Is(0) (e^(q*Vbe/nelkT) -1) I changed the value of C2 and nel to describe the nonideal component of Ib which is dominant at low currents.So let me know if i am going wrong and please refer me to someone who knows about this.

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Geoffrey_Coram

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Re: Pspice BJT Modeling
Reply #1 - Jul 29^th, 2005, 7:57am

Without seeing the whole set of equations, it looks like you're on the right track.
Typically, the base current has an ideal term
Ib(ideal) = Is * (exp(vbe/vt) -1)
and a non-ideal term
Ib(nonideal) = Isn * (exp(vbe/(2*vt) -1)

where Isn is bigger than Is, so that it dominates at low vbe, but at high vbe,
the factor ne1 (usually ~2) means that the exponential is smaller. With more base current, the gain ic/ib is smaller.

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viren

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Pspice BJT Modeling
Reply #2 - Jul 29^th, 2005, 10:33pm

Thank you for your suggestion. I am modeling a non ideal BJT characteristics. I am using the gummel poon model parameters for pspice . I just have a graph of the real BJT DC characteristics for IC Vs Vce curve. I changed the value of ISE so that there is a drop in gain at low IC current but when I change the value
of IS my max Ic current for the graph goes above 5 A which is wrong. Can u tell me what mistake i am doing to calculate gain at high IC current .

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Geoffrey_Coram Senior Fellow Offline Posts: 1998 Massachusetts, USA	Re: Pspice BJT Modeling Reply #3 - Aug 1^st, 2005, 5:29am Why are you changing IS? You changed ISE for low current, did you also need to fix the high current end? In real BJTs, the high current regime may be limited by the resistances (RC, RE, RB).
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