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Old 20th November 2002, 11:05 AM   #11
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Default Need help etc

Bricolo,

You need help? You're kidding..!

Chapeau! (Couldn't find the smiley that takes its hat off)

Jan Didden
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Old 20th November 2002, 07:09 PM   #12
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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.
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Old 20th November 2002, 07:53 PM   #13
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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
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Old 20th November 2002, 07:55 PM   #14
Bricolo is offline Bricolo  France
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Nelson: I found nothing about transconductance in the 2SK's datasheet
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Old 20th November 2002, 08:32 PM   #15
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Lightbulb 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
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Old 20th November 2002, 08:48 PM   #16
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Quote:
Originally posted by Bricolo
Nelson: I found nothing about transconductance in the 2SK's datasheet
The curve you posted and the region you indicated gives
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

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Old 21st November 2002, 05:16 PM   #17
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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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