Hello,
I always wanted to know how to predict the value of the AC gain in an Aleph according to the current source "feedback" resistors (R12 and R15 in the attached image for example). I couldn't find it anywhere on this forum so I finally decided to do it myself the old way on a piece of paper with a pen. It was worth because I understood the quite tricky behavior of the aleph current source which responds to the modulation of the lower mosfets by a sum of positive and negative feedback (yes, I said positive) the later being of course stronger.
I wanted to share with you for comments and to help the people not having an oscilloscope and a sinewave source at home (many people I guess).
My calculation is assuming the output resistance (R20 and 21in parallel in my example) is half the one of the current-source source resistor in parallel (R18 only here) which is the usual topology. It works whatever the number of fets is if this condition is respected.
We will call : B=R12/R15 (so it’s less or equal to 1)
and : A=AC gain
B = 1 / (2xA)
For information :
Imax = Ibias x 2B/(2B-1) = Ibias / (1-A)
but it was already available in the Aleph power xls file.
I hope you will find it useful : I think it’s always better to have a real resistor instead of a trimpot. You just pick your value (or values you can choose from with a jumper) and go… no more trial and error for that!
I think I made no mistakes... Waiting for your comments people...
Bye
I always wanted to know how to predict the value of the AC gain in an Aleph according to the current source "feedback" resistors (R12 and R15 in the attached image for example). I couldn't find it anywhere on this forum so I finally decided to do it myself the old way on a piece of paper with a pen. It was worth because I understood the quite tricky behavior of the aleph current source which responds to the modulation of the lower mosfets by a sum of positive and negative feedback (yes, I said positive) the later being of course stronger.
I wanted to share with you for comments and to help the people not having an oscilloscope and a sinewave source at home (many people I guess).
My calculation is assuming the output resistance (R20 and 21in parallel in my example) is half the one of the current-source source resistor in parallel (R18 only here) which is the usual topology. It works whatever the number of fets is if this condition is respected.
We will call : B=R12/R15 (so it’s less or equal to 1)
and : A=AC gain
B = 1 / (2xA)
For information :
Imax = Ibias x 2B/(2B-1) = Ibias / (1-A)
but it was already available in the Aleph power xls file.
I hope you will find it useful : I think it’s always better to have a real resistor instead of a trimpot. You just pick your value (or values you can choose from with a jumper) and go… no more trial and error for that!
I think I made no mistakes... Waiting for your comments people...
Bye
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