fT vs ic of BJTs


2007-12-19 8:53 pm
The fT/ic characteristics of power BJTs is rarely published and if, then logarithmic scale for fT because it is close to horrible.
Some widely used fast power BJTs start with a mere fT of 100 kHz at ic= 50 mA
and reach fT 60 MHz at ic= 3 amps.
In terms of compensation, effect of nfb, reactive load this means designer have to take the lowest fT into account.
While the effect can be easily understood from both technology ( such as ring emitter multi-emitter etc.) and semiconductor physics, it is hardly ever mentioned.
Good old tubes, which are charge amplifiers while BJT are current amplifiers,
display such an effect, too, but not in such an order of magnitude, and are generally much faster.
For power Mos : fT = gm/(2*pi*Cin)
and also here gm and Cin are dependent of drain current. It should be so that
Mosfets are also significantly slower at low Idrain.

Mark Johnson

Paid Member
2011-05-27 3:27 pm
Silicon Valley


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