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Old 22nd November 2012, 06:27 PM   #3021
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so - having degeneration because of tempco , while not sacrificing xconductance

ZAP per personal choice
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Old 22nd November 2012, 06:39 PM   #3022
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Nelson, any experience with the Cree parts?
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Old 22nd November 2012, 07:51 PM   #3023
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Quote:
Originally Posted by lhquam View Post
Rs=0R47 sounds about right for thermal stability.
Any reason not to use one or more "Zen pots"?
1.0 ohms would be even better, unless you use some temperature
compensation. The technique of F5 would be an example.

No reason not to use "Zen pots" - on one extreme, AC degeneration, on the
other extreme, my suggestion above.

Of course some laterals will solve the temperature coefficient issue, but will
have a lot lower transconductance. Still, they'll work fine - as I said earlier,
we have open loop gain to burn.

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Old 22nd November 2012, 08:39 PM   #3024
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EUVL and others have implemented F5s without thermistors with Rs as low as 0R22 and claim that they are thermally stable. I suppose it could also depend on the choice of MOSFET.

Quote:
Originally Posted by Nelson Pass View Post
1.0 ohms would be even better, unless you use some temperature
compensation. The technique of F5 would be an example.

No reason not to use "Zen pots" - on one extreme, AC degeneration, on the
other extreme, my suggestion above.

Of course some laterals will solve the temperature coefficient issue, but will
have a lot lower transconductance. Still, they'll work fine - as I said earlier,
we have open loop gain to burn.

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Old 23rd November 2012, 12:32 AM   #3025
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The IRFP240's have a lower thermal coefficient than the Fairchild parts in the
F5, so that is plausible.

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Old 23rd November 2012, 12:55 AM   #3026
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How come all the semiconductors we want to use have been discontinued but the IRFP240s are still being made?
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Old 23rd November 2012, 01:03 AM   #3027
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I think EUVL's F5X uses Toshiba 2SK1530/2SJ201 MOSFETs.

I am considering Fairchild FQA28N15 MOSFETs in the F6. Using the Fairchild PSPICE models I see some rather strange results for the 3rd harmonic shown below. I do not understand why H3 is so flat over frequency. Until I measure in an actual circuit I cannot trust the FQA28N15 SPICE models. I have been able to get pretty good correspondence between SPICE and measurements for the SemiSouth R100s.


Quote:
Originally Posted by Nelson Pass View Post
The IRFP240's have a lower thermal coefficient than the Fairchild parts in the
F5, so that is plausible.

Attached Images
File Type: jpg FQA28n15-2.jpg (82.6 KB, 384 views)
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Old 23rd November 2012, 01:09 AM   #3028
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Ilquam,
I might have some of those as well as the FQA 32n20c, if you want them. I looked at them as alternatives to the 19n20c in the GB.
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Old 23rd November 2012, 01:17 AM   #3029
lhquam is offline lhquam  United States
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Thanks for the offer, but I purchased 50 each FQA28N15 and FQA36P15 a couple of years ago and measured them for Vgs and Gm at 1.3A. If you need some matched pairs, I have them.

BTW: The SPICE sim show a transconductance of 6.6S at 1.4A for the FQA28N15 vs. 7.8S for the SemiSouth R100s. Pretty good!

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Ilquam,
I might have some of those as well as the FQA 32n20c, if you want them. I looked at them as alternatives to the 19n20c in the GB.

Last edited by lhquam; 23rd November 2012 at 01:22 AM.
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Old 23rd November 2012, 01:42 AM   #3030
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Reason i mention FQA32n20c is that i believe it is higher.
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