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Simulators >> Circuit Simulators >> Ft in ADE->Results->Print->DC Operating Points?
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Message started by ayowchen on Aug 22^nd, 2006, 6:43am

Title: Ft in ADE->Results->Print->DC Operating Points?
Post by ayowchen on Aug 22^nd, 2006, 6:43am

Hi all,

We are using both Gummel-Poon and HICUM. And Ft in ADE->Results->Print->DC Operating Points is frequently used.
Does anyone know how this ft comes from?

Such ft result is actually inconsistent with the freq @ H21=1 from AC simulation. :o

Many thanks.
Ayow

Title: Re: Ft in ADE->Results->Print->DC Operating Points
Post by Geoffrey_Coram on Aug 22^nd, 2006, 1:01pm

fT in the Gummel-Poon model is gm/(2 pi * (Cmu + Cpi)) and is a lousy approximation.

The Hicum model defines a somewhat more sophisticated result, but Mextram has the best derivation of fT using all the partial derivatives of the various quantities, considering the non-idealities and internal nodes (internal base, collector) of the model.

That said, most people don't actually look at the frequency where H21=1, because if you have a zero in the response, it may level off instead of crossing 0dB. You're better off using the Gummel formula freq / imag(1/H21) at a frequency slightly above the 3dB point (the corner frequency).