Since I'm in the process of buying the final parts for my new Aleph30 clone, I have a question:
1. How does the transconductance (Gfs) of the output FETs affect the current gain?
I'm asking, because I am going to use IRFP140A from Fairchild, see http://www.fairchildsemi.com/ds/IR/IRFP140A.pdf
and this fet has a Gfs of about 23, versus the original IRF244 has about 6.7.
The A30 has a (*theoretical, not taking Gfs into consideration) Gain setting of about 61%.
2. What would be the better(*) gain figure with my chosen FETs?
(I do not have the equipment to measure it, sorry
)
1. How does the transconductance (Gfs) of the output FETs affect the current gain?
I'm asking, because I am going to use IRFP140A from Fairchild, see http://www.fairchildsemi.com/ds/IR/IRFP140A.pdf
and this fet has a Gfs of about 23, versus the original IRF244 has about 6.7.
The A30 has a (*theoretical, not taking Gfs into consideration) Gain setting of about 61%.
2. What would be the better(*) gain figure with my chosen FETs?
(I do not have the equipment to measure it, sorry
)
Mads,
if you have a fixed voltage at the gate, the Gfs determines
the outputresistance of that fet. In the Aleph case this is
not true as the gate is driven by a transistor to form a
constant current source, as the beta of that trannie is about
200 times as high as the Gfs of the fet, the Gfs plays just a
minor part in that equation. As the CCS is "modulated" by
the sensed outputsignal the current gain only depends
on the amount of that modulation.
BTW using 12 fets with say Gfs = 6 or 6 fets with a Gfs
of 12 is exactly the same, as the Gfs is the sum of all Gfs´s
of the paralelled fets.
Uli
if you have a fixed voltage at the gate, the Gfs determines
the outputresistance of that fet. In the Aleph case this is
not true as the gate is driven by a transistor to form a
constant current source, as the beta of that trannie is about
200 times as high as the Gfs of the fet, the Gfs plays just a
minor part in that equation. As the CCS is "modulated" by
the sensed outputsignal the current gain only depends
on the amount of that modulation.
BTW using 12 fets with say Gfs = 6 or 6 fets with a Gfs
of 12 is exactly the same, as the Gfs is the sum of all Gfs´s
of the paralelled fets.
Uli
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