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Old 5th March 2004, 03:48 AM   #141
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Default Incresaed Bias on the Diff Pair.

Forgive me for not bookmarking the post and linking it here, but I was not thinking when I saw it. And I havn't been able to find it again. I can't remeber what I used for search topic.

In my searching, I have found that there has been quite a bit of discusson about increasing the bias. I also saw a short post contemplating why Nelson originally chose the 400 ohms of the load resistor. Paraphrasing the comment from I believe Mr Thurgard, '400 ohms seems to give the maximum amount of gain for a given bias to achieve proper bias on the gain devices' he went on to say something well beyond my ability to understand that lowering the value of the load resistor and increasing the bias is exceeded by the change in transconductance.

If that is true, then the 400ohm would be close to optimal for the characteristics of the devices used in this circuit.

Could anybody with a more fundemental grasp on this help to clarify?

Thanks
Scott.
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Old 5th March 2004, 03:49 AM   #142
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After you see the prices, you may not be so eager to overkill the heatsinks by much.
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Old 5th March 2004, 01:34 PM   #143
hifiZen is offline hifiZen  Canada
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Quote:
Originally posted by moe29
... Of course this means they're not mounted
back to back during testing...
Hmm. Not to put a fly in the ointment, but they really ought to mounted together if you can manage it... junction temperature affects a MOSFET's behaviour, so you need to be sure they're at exactly the same temp when you do the matching. This is the reason they're mounted back to back on the PCB - the tight thermal coupling keeps the die temperatures very close, and their Vgs drift matched as closely as possible during operation.

You can also do as Grey says, and swap Q2 and Q11 around to take advantage of their Vgs mismatch...
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Old 5th March 2004, 02:44 PM   #144
moe29 is offline moe29  United States
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i took a day off from working on the circuit, and i think i'll try
swapping them around, with back to back mounting today.
That seems to be the reasonable approach, as apposed to let's
get it done as fast as possible!
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Old 8th March 2004, 04:10 PM   #145
Coulomb is offline Coulomb  England
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Can anybody explain the difference between K/W used in the AXE spreadsheet and C/W used by Extrusion manufacturers?

Is it Kelvin/Watt?

I know C/W is Celsius/Watt.

How do I convert from one to the other?

Regards

Anthony
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Old 8th March 2004, 04:17 PM   #146
Netlist is offline Netlist  Belgium
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1C = 1K
K starts at -273.15C
http://www.2spi.com/spitemp.html
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Old 8th March 2004, 04:18 PM   #147
UrSv is offline UrSv  Sweden
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It's the same with respect to the heatsink data.
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Old 8th March 2004, 06:04 PM   #148
Coulomb is offline Coulomb  England
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Thanks Guy's
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Old 8th March 2004, 08:15 PM   #149
Coulomb is offline Coulomb  England
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For those of us in the Frozen North (Canada) check out this thread if you thinking of buying some Heat Sinks.

Regards

Anthony

http://www.diyaudio.com/forums/showt...193#post344193
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Old 9th March 2004, 04:33 AM   #150
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Ahhh, The things we do when we get tired... I mistakenly posted this under the Electronics and parts heading...... So I',m moving it to where it belongs.....

I'm about to run up my first X board on the variac and was wondering whats the proper procedure for setting the input pair current source adjustment???

Thanks,
Mark
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