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20th November 2002, 08:09 PM  #12 
The one and only

I can't tell if you figured this part out: The output impedance
of the Source pin is the inverse of the transconductance of the Mosfet, and from your curve we see the transconductance to be about .2 amps per volt, for an output impedance of about 5 ohms. Your bias resistor plus headphone will load this down, and it would appear that they represent something like 15 ohms, forming a voltage divider that gives a .75 gain. 
20th November 2002, 08:53 PM  #13 
diyAudio Member
Join Date: Nov 2002
Location: Grenoble, FR

I didn't understant everything you told, Nelson
I'll try to find more infos about this transconductance I didn't take care of the mosfet own resistance. The yellow line is only calculated by 1/R4 I've just done some mesures with the CCS modified circuit. I've got some weired results My V4=f(Vg) graph is a line going up slowly, still Vg=6V from Vg=6 to 12, it is a line, going up faster I think the 1st part is when the LM317 hasn't reached 250mA (I set it to 250mA) The lm317 is not a perfect current source it's more like a current limiter that explains why we got 2 part for the V4=f(Vg), a first one when the 317 hasn't reached 250mA, and another one when it limits the current to 250mA I can't mesure the current to prove this, but I'm quite sure it's that 
20th November 2002, 08:55 PM  #14 
diyAudio Member
Join Date: Nov 2002
Location: Grenoble, FR

Nelson: I found nothing about transconductance in the 2SK's datasheet

20th November 2002, 09:32 PM  #15 
diyAudio Member
Join Date: Jan 2002
Location: Earth

general definitions
Reciprocals:
Resistance Conductance Reactance Susceptance Impedance Admittance Ratios: V/V voltage gain, no units V/I resistance or impedance, ohms I/V conductance or admittance, mhos or siemens I/I current gain or beta, no units A trans[ratio] amplifier converts ratio denominator (input) to the ratio numerator (output). Eg: transconductance of a FET is I(out)/V(in) or Id/Vgs 
20th November 2002, 09:48 PM  #16  
The one and only

Quote:
us the information. .5 volt difference Gate to Source gives another 100 ma Drain current. This is a 200 mA/volt figure, and becomes .2 A/V 

21st November 2002, 06:16 PM  #17 
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Join Date: Nov 2002
Location: Grenoble, FR

Nelson
I made no calculations about the mosfet transconductance just: 12=Vds+R.I so I changes with 1/R that's all you see on my xls file, on I=f(Vds) the line's equation is 45 45/100 (mA) is equal to 1/22Ohm 
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