F5X -- the EUVL Approach - Page 41 - diyAudio
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Old 21st August 2011, 01:03 PM   #401
JBdV is offline JBdV  Netherlands
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Tnx Patrick, I have just asked him.
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Old 22nd August 2011, 05:02 AM   #402
EUVL is offline EUVL  Europe
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I have been asked why there is a need to match for transconductance, when we already matched tightly for Idss.

The attached graph are two 2SJ74BLs with identical Idss, actually measured with a curve tracer.
As you can see, real devices do not obey theoretical equations.

To match for transcoductance for F5 without a curve tracer, the following procedures are sufficient :

2SK170BL -- measure Id with 0R, 11R, 22R degeneration
2SJ74BL -- measure Id with 5.1R, 16R, 27R degeneration

Devices (NNPP) with identical values for all 3 columns are best matched for the F5X.


Patrick

PS These procedures will be described in more detail by the test team eventually

.
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Old 22nd August 2011, 05:23 AM   #403
ashaw is offline ashaw  Australia
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For the benefit of others, can you draw a schematic of the resistor degeneration and jfet for Id measurement?

Is this accurate?
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Old 22nd August 2011, 10:25 AM   #404
EUVL is offline EUVL  Europe
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Yes, and the top rail is positive for N-FET, negative for P-FET.


Patrick
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Old 23rd August 2011, 10:38 AM   #405
JBdV is offline JBdV  Netherlands
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Patrick, you have email (through this website)
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Old 23rd August 2011, 12:46 PM   #406
AndrewT is offline AndrewT  Scotland
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post403:
for an ammeter use a 240r 0.1% resistor and a 200mVdc DVM with a 0.1% scale accuracy. This is likely to be 100 times more accurate than using an ammeter.
Or settle for 10 times more accuracy using 1% devices.
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Old 23rd August 2011, 12:49 PM   #407
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How important is the stability of the voltage source in this process ?

D.
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Old 23rd August 2011, 12:53 PM   #408
AndrewT is offline AndrewT  Scotland
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the standard test is Vds=10V and Tc=25degC.
If either of those vary, then your results will be different.
A battery may be stable enough if it is fresh, but be very wary of drooping voltage if the test session is extended.
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Old 23rd August 2011, 02:18 PM   #409
EUVL is offline EUVL  Europe
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I use regulated supply. So the supply was not an issue.

Temperature is. Breathing on it will swing the Id by as much as 50uA.

So although my ammeter can measure to 1uA, I never quote better match than 20uA.
If you connect the JFET and just watch it from 2m distance over 5 minutes, it will still be drifting to within 10-20uA.


Patrick
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Old 23rd August 2011, 03:56 PM   #410
Horio is offline Horio  United States
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Quick (newbie) question regarding gain. From a post in the old thread, it sounds like the gain of the F5X will be the same as the F5 (~5.6 or 15dB). If I am running the amp with a simple buffer with no gain for a preamp, how many Volts (Vrms) does my source need to output to push the amp to its maximum power output?
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