Jfet folded cascode with diamond buffer opamp - diyAudio
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Old 21st May 2011, 01:22 PM   #1
sonnya is offline sonnya  Denmark
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Default Jfet folded cascode with diamond buffer opamp

Hi all.

Well i thought it was time to do some new stuff.

So my first attempt is to make an opamp replacement with jfet inputs.

I will try to hit the 700x700mil size for this one.

This will be fun.

The jfet has an input noise level of 6nV so it will be able to be used for riaa as well as linelevel.

I have attached the diagram. I expect to have the board ready by tomorrow.
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File Type: pdf jfetopampv10.pdf (29.3 KB, 1142 views)
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Old 21st May 2011, 02:15 PM   #2
jam is online now jam  United States
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Hi Sonnya,

Nice circuit.

Question . Does cascoding the folded cascode help much? I would think it's use in this application would be minimal but I could be wrong.
I like the use of the Wilson Mirror which I believe is far better than the Widlar that is often used.

Keep up the good work.

Regards,

Jam

Last edited by jam; 21st May 2011 at 02:17 PM.
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Old 22nd May 2011, 04:04 AM   #3
sonnya is offline sonnya  Denmark
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Hi Jam.

Thanks for the comment.

The widlar currentsource is made for the silicon process as it is not possible to make high ohm resistors in the silicon. By adding the resistor in "mirror" transistor it is possible to compensate for it. Correct me if i am wrong.

I can see that i maybee has been "blinded" by the use of folded cascode as i have used them before.

I would be easy to replace it by two wilson mirror and it would require the same or less transistor. It would actually raise the impedance in the tranconductance stage, like i wanted it to.

So i will change that part. thanks for the guiding question.
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Old 22nd May 2011, 04:28 AM   #4
jam is online now jam  United States
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Sonnya,

Actually I like the four transistor Wilson Mirror (Still might be a bit of overkill).
I will not disagree with you that the folded cascode can sound very good and only requires matching of the diff and current mirror offers equal pull-up and pull-down. All in all a very elegant circuit.
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File Type: gif Imp_wilson_cs.gif (2.5 KB, 1134 views)

Last edited by jam; 22nd May 2011 at 04:31 AM.
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Old 22nd May 2011, 04:35 AM   #5
sonnya is offline sonnya  Denmark
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Yes i know it, but i think i will not be able to get it down onto 700x700mil board. It is already a plenty booked board.
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Old 22nd May 2011, 04:38 AM   #6
jam is online now jam  United States
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Sonnya, if you are making boards I would like to purchase a few if possible......so would a few other members, I bet.

Regards.

Jam
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Old 22nd May 2011, 01:23 PM   #7
sonnya is offline sonnya  Denmark
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Default final layout.

Here it is.

The size of the module is 800x800mil or 20.32x20.32mm.

More info will come.
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File Type: jpg DPADS ProjectsSITO Audioopampjfetopampv10.pcb - PADS Layout 22-05-2011 142021.bmp.jpg (624.8 KB, 1183 views)
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File Type: pdf jfetopampv10.pdf (28.5 KB, 209 views)
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Old 24th May 2011, 03:56 AM   #8
Bonsai is offline Bonsai  Taiwan
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Sonnya, have you simmed it to see what the performance is like? Looks like a an interesting circuit.
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Old 25th May 2011, 06:27 AM   #9
sonnya is offline sonnya  Denmark
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I will sim it this weekend.
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Old 27th May 2011, 06:05 AM   #10
mlloyd1 is offline mlloyd1  United States
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indeed ...

mlloyd1

Quote:
Originally Posted by jam View Post
Sonnya, if you are making boards I would like to purchase a few if possible ...... so would a few other members, I bet.

Regards.

Jam
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