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Old 30th March 2008, 08:08 PM   #11
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Wow, nice job!
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Old 30th March 2008, 08:17 PM   #12
EUVL is offline EUVL  Europe
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Aren't they cute !!

(Not yet potted, or else you will see nothing much .... )




Patrick
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Old 30th March 2008, 08:21 PM   #13
EUVL is offline EUVL  Europe
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In case you ask how it sounds -- I don't know, because all my equipment have been so modified in the years that they don't have opamps in the signal path anymore !!

But try it for yourself.
It is very interesting project -- the building will test your DIY skills.




Patrick
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Old 30th March 2008, 08:40 PM   #14
EUVL is offline EUVL  Europe
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Oh, speaking of manners. How dare I forget :

Most sincere thanks to Bertram for making the PCBs, and Markus for Spice simulations.

Great team.


Patrick
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Old 31st March 2008, 12:56 AM   #15
mlloyd1 is offline mlloyd1  United States
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way cool!

mlloyd1
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Old 31st March 2008, 01:04 AM   #16
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Hi Patrick,

That's very nice work.

Must work on my soldering skills...

Cheers,
Dennis
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Old 31st March 2008, 01:26 AM   #17
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Very cool!


hats off,
7/10
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Old 31st March 2008, 05:38 AM   #18
EUVL is offline EUVL  Europe
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A few additional remarks perhaps.

The circuit is pretty standard, as published by Nelson & others. The changes made were more out of necessity than desire, though I think the BF862 in this case is arguably a better part. The only "disadvantage" is the high bias. But then the high bias allows you pure Class A output (Class B not possible anyhow) of +/- 15mA.

If your application does not support the 40mA it consumes, you can either use BF861 instead (they come in 3 grades of Idss), or with slight changes to the PCB use LSK170s.

Soldering is tricky because of the tight space, but not more difficult than any other SMDs. Matching SOT-23s is a very interesting experience. Do put a current limiter of say 30mA on your power supply. Once setup, it took me an hour to measure 100 devices.

The most difficult is the layout. It looks simple now, but it was a week's work and 7 versions before it gets to this state. As you can see, there isn't really much room left for more complicated circuits like current mirrors or folded cascodes, ...., etc. There is, however, room on the PCB for a ceramic compensation cap in 0603 (e.g. Murata). There is no small silver mica SMDs I can find on the internet. So I myself would solder it external.

The DC offset is thermally sensitive. You need to trim DC offset using the actual supply voltage. Once it stablises, especially with the heatsink, it remains very stable. I could get DC stable to within 2mV over time on a close loop gain of 11.

The compensation cap also depends on the close loop gain. The higher the gain, the smaller the cap, as expected. I found that I need to use 220p for a gain of 5. Again, recommend testing in a dummy setup close to the actual application. I have no problems finding small enough silver micas to allow direct soldering onto the 2 adjacent pins (Pin 6 & 8). Similarly, additional trimming resistors can be soldered externally between Pins 7 & 8.

You can make 10 of these for less than 100 Euros, buying parts from RS and making your own PCB. Heatsinks cost about 1 Euro each from Buerklin, as are the 0.8mm pins (20 cents each). Not bad at all considering you have to pay similar or more for an OPA627, LME49710, or even AD797. The price will drop by a factor of 4 if you order 1000 FETs from Digikey. But that I suppose is something only for group buys.




Patrick
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Old 31st March 2008, 05:59 AM   #19
steenoe is offline steenoe  Denmark
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Quote:
Originally posted by EUVL
Aren't they cute !!

(Not yet potted, or else you will see nothing much .... )




Patrick
They are cute, indeed

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Old 31st March 2008, 09:24 AM   #20
EUVL is offline EUVL  Europe
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Steen,

If you have an application to test, and promise to publish the results, send me a private mail (you have the address).




Patrick
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