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https://designers-guide.org/forum/YaBB.pl Modeling >> Semiconductor Devices >> gm of BJT https://designers-guide.org/forum/YaBB.pl?num=1126195500 Message started by Geoffrey_Coram on Sep 8th, 2005, 9:05am |
Title: gm of BJT Post by Geoffrey_Coram on Sep 8th, 2005, 9:05am I'm working with a BJT (doesn't really matter what model), and trying to understand the value of gm. According to first-order theory, gm = Ic/Vt. gm is really defined as the partial derivative of Ic with respect to Vb. Now, if I do a sweep of the base voltage, I can plot Ic/Vt and derivative(Ic) on the same graph, but they don't agree for high base voltage (off by a factor of 2). Ic/Vt does match the "gm" reported by the device fairly well. I've tried turning off self-heating, and I've checked that there isn't much base current (Vbe - Ib * Rb hardly differs from Vbe) and there also isn't much avalanche current. What else needs to be considered? |
Title: Re: gm of BJT Post by Geoffrey_Coram on Sep 8th, 2005, 10:25am OK, Ib is small, but Ie isn't! So, Vbe - Ib*Rb - Ie*Re is 28mV smaller than Vbe, which is pretty significant. It's a small-geometry device. |
Title: Re: gm of BJT Post by sheldon on Sep 17th, 2005, 1:42am Geoffrey, The gm is in series with the Re of the transistor at high currents so the effective transcondcutance is limited by the Re of the transistor. In the earlier days of polysilicon emitters, this effect was one of the concerns about migrating to polysilicon emitters along with the stability of the interface. Best Regards, Sheldon |
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