"Composite Op Amp For High Performance," Siliconix AN81-3, July 1983

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mediatechnology
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"Composite Op Amp For High Performance," Siliconix AN81-3, July 1983

Post by mediatechnology » Fri May 17, 2019 3:44 pm

"Composite Op Amp For High Performance," Siliconix AN81-3, July 1983

This 1983-era application note from Siliconix shows a differential FET front-end connected to the compensation and offset adjustment terminals of an NE5534.

"Composite Op Amp For High Performance," Siliconix, AN81-3, July 1983, pdf: https://proaudiodesignforum.com/images/ ... y_1983.pdf

Image
"Composite Op Amp For High Performance," Siliconix, AN81-3, July 1983.
Note: An error in the original drawing has been corrected to add dots to connect pin1 to the collector of the inverting input transistor and pin 7 to the NE5534 internal Vcc node.
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Re: "Composite Op Amp For High Performance," Siliconix, AN81-3, July 1983

Post by JR. » Fri May 17, 2019 4:28 pm

That sounds familiar, I might have seen that in a trade magazine, back then.

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Re: "Composite Op Amp For High Performance," Siliconix, AN81-3, July 1983

Post by mediatechnology » Fri May 17, 2019 5:12 pm

I just noticed that there's a dot missing in the schematic to connect pin 1 to the collector of the inverting input transistor.
As drawn the Drain loads aren't both in parallel with the Collector loads.

EDIT: I went back and edited the thumbnail and pdf to correct the original drawing error.
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Re: "Composite Op Amp For High Performance," Siliconix, AN81-3, July 1983

Post by billshurv » Fri May 17, 2019 5:24 pm

I've always wondered about making one of these as it gets mentioned quite a bit, but not seen the article before. However given some of the really nice FET input opamps available now is there any real reason to do it other than fun?

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Re: "Composite Op Amp For High Performance," Siliconix, AN81-3, July 1983

Post by JR. » Fri May 17, 2019 6:54 pm

IMO not really, now... maybe back then a little, but even back then the stock 553x and TL07x were better than needed for most line level audio paths. That was an inflection point for audio design when the off the shelf mass market ICs did not suck. 8-)

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Re: "Composite Op Amp For High Performance," Siliconix, AN81-3, July 1983

Post by mediatechnology » Fri May 17, 2019 7:14 pm

I've had the Siliconix July 1983 Small FET Design Catalog for about 35 years and had never noticed that short app note.

I had heard about it before and when I ran across it I set it aside.
Found some references to AN81-3 but no web presence.
Thought it might be a good idea to post it for prosperity.

One application today that might make sense for low noise would be to pair BF862 to the NE5534.
It would likely excel over most FET op amps in the noise department and would be reasonably low cost as the BF862 are cheaper than dirt.
If I'm not mistaken the NE5534 and most single audio op amps have almost become sole-sourced now by TI with NJR discontinuing the 5534.
That's the downside.

Would the cascode connection of the input FETs also reduce CM distortion from delta Cgs compared to most FET input op amps?
Dimitri Danyuk and Douglas Self both have done a lot of work with bootstrapping to lower op amp distortion.
Self shows us bootstrapped TL072s; Danyuk bootstraps the bipolar LME49710 to make it work like a high impedance FET amp.
Those circuits have a lot more outboard bits than 4X BF862 + current sink + NE5534.

ZTX851 + NE5534 in this topology might also be fun to try for a low-noise bipolar building block.

I posted it mostly for its historical interest.
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Re: "Composite Op Amp For High Performance," Siliconix, AN81-3, July 1983

Post by billshurv » Fri May 17, 2019 7:44 pm

Well I do have a few BF862s hanging around so if I don't use them all up on other things got to be worth a try. I should have got more when the EOL notification came out, but was too backlogged on projects to think it was a good idea. Luckily there are some other parts that are almost as good in the onsemi catalogue and still in production.

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Re: "Composite Op Amp For High Performance," Siliconix, AN81-3, July 1983

Post by mediatechnology » Fri May 17, 2019 7:55 pm

Looks like the BF861 is still around but its 1.5 nV√Hz vs. 0.8 nV√Hz for the '862.
I had thought that Nexperia had kept the 862.
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Re: "Composite Op Amp For High Performance," Siliconix AN81-3, July 1983

Post by billshurv » Sat May 18, 2019 5:24 am

Of course I made a very detailed note of a good replacement and saved it somewhere so safe I can't remember. If I find it I'll let you know and we can pop it somewhere where a list of good low noise FETs would be useful.

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Re: "Composite Op Amp For High Performance," Siliconix AN81-3, July 1983

Post by mediatechnology » Sat May 18, 2019 5:54 am

You could add it here: viewtopic.php?f=12&t=1136

I thought I had posted Dimitri's n-FET noise rankings before but I couldn't find it. I made a pdf of it since Electronic Design and EDN are notorious for unintentionally scrubbing figures from articles as the magazine is repeatedly bought and sold.
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