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Old 30th January 2003, 12:35 PM   #1
Mad_K is offline Mad_K  Norway
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Question Aleph current gain and transconductance

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 )
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Old 30th January 2003, 02:59 PM   #2
uli is offline uli  Austria
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Hello Mads,
the Siemens figure doesn´t change the gain.
The relationship between sensed voltage across the source
resistor and the sensed voltage across the output resistor(s)
defines the gain!

Uli
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Old 30th January 2003, 03:05 PM   #3
Mad_K is offline Mad_K  Norway
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I know, but I also know that the Gfs also plays a part here. This is why I'm talking about theoretical vs. real figures. I guess The One And Only have to enlighten us...
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Old 30th January 2003, 03:18 PM   #4
uli is offline uli  Austria
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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
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Old 30th January 2003, 04:25 PM   #5
Mad_K is offline Mad_K  Norway
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Yepp, I guess it doesn't matter that much. My guess is that the CG goes up a few %... No biggy.

Thanks for your comments
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Old 30th January 2003, 04:28 PM   #6
uli is offline uli  Austria
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Quote:
Originally posted by Mad_K
Yepp, I guess it doesn't matter that much. My guess is that the CG goes up a few %... No biggy.

Thanks for your comments
Nevermind
I normally use the cheapest available Mosfets

Uli
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