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Old 19th April 2017, 03:54 AM   #1
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Default Fully complementary AD797?

An AD797 thread had this post, some years back.
Quote:
Originally Posted by scott wurcer View Post
...I still would like to make a fully compl[e]mentary version.
I have never seen a follow-up but the idea still has appeal.
Anyone have any ideas on the best way to do this?
Best of all if Scott could show his own idea

David
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Old 19th April 2017, 04:43 PM   #2
PB2 is offline PB2  United States
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I've never been a big fan of fully comp since you have to match N to P type and they
never really match due to device physics. I also do not like a single ended VAS with
a current source on the other side which leads me to topologies where the other diff
pair output is used to drive the other side of a push-pull VAS. There are many examples
such as the APT power amp, several from Harman Kardon, and this Onkyo just as an
example - the front end to the VAS, nothing more:
Thermal compensation for super complementary output stage?

What do you see as the reasons to go full comp?

Another advantage of a single ended diff pair is that many FETs are more available in
N type only.

Curious to see what Scott has to say.
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Old 19th April 2017, 11:46 PM   #3
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Quote:
Originally Posted by PB2 View Post
...What do you see as the reasons...
One of the benefits should be even lower noise.
Additional input transistors in parallel is a classic way to lower Vnoise.
A complementary IPS pair is effectively a way to do this but has the benefit that input bias currents are cancelled rather than added.
Whereas the AD797 has an input bias cancellation circuit that inevitably adds some noise, as Scott himself has pointed out. The datasheet published by AD is very quiet on the subject, which is appropriate I suppose.

It should be easier to achieve cancellation of current induced distortion and hopefully some parasitics may cancel.
And, while I understand that the physics prevents accurately complementary devices, symmetry does look nice.

Quote:
Curious to see what Scott has to say.
Me too, it seems people are a bit hesitant to post before they read his comments.

Best wishes
David

Last edited by Dave Zan; 19th April 2017 at 11:48 PM.
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Old 20th April 2017, 03:16 AM   #4
PB2 is offline PB2  United States
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JFETs solve the input bias issue, yes?
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Old 20th April 2017, 04:49 AM   #5
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Quote:
Originally Posted by PB2 View Post
JFETs solve the input bias issue, yes?
Yes, but the lowest noise inputs are still BJT, I believe the new edition of "The Art of Electronics" has the best information on this, I need to buy a copy.
And a complementary self-biased JFET CFA is also a nice solution.
The complementary input JFET saves a VAS current source or an APT style current mirror so it's actually simpler, with lower noise as a bonus if it's useful.

Best wishes
David
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Old 20th April 2017, 05:55 AM   #6
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Quote:
Originally Posted by Dave Zan View Post
Yes, but the lowest noise inputs are still BJT, I believe the new edition of "The Art of Electronics" has the best information on this, I need to buy a copy.
And a complementary self-biased JFET CFA is also a nice solution.
The complementary input JFET saves a VAS current source or an APT style current mirror so it's actually simpler, with lower noise as a bonus if it's useful.

Best wishes
David
Fully symmetric complementary (not similar to AD797 and no GNFB) jfet preamp. Lastly I build symmetric complementary amps only. I like to look the schematic, but I like the sound too.
BR Damir
Current conveyor as a voltage amplifier
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Old 20th April 2017, 07:00 AM   #7
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Quote:
Originally Posted by dadod View Post
Fully symmetric complementary (not similar to AD797...
Hi Damir
Yes, I noticed that circuit of yours, a self-biased JFET CFA input has a lot of benefits.
But the AD797 has some clever ideas and I can't easily see a way to do a fully complementary version so I am curious about Scott's post.
Can you have an idea for this circuit?

Best wishes
David
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Old 20th April 2017, 08:22 AM   #8
dadod is offline dadod  Croatia
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Quote:
Originally Posted by Dave Zan View Post
Hi Damir
Yes, I noticed that circuit of yours, a self-biased JFET CFA input has a lot of benefits.
But the AD797 has some clever ideas and I can't easily see a way to do a fully complementary version so I am curious about Scott's post.
Can you have an idea for this circuit?

Best wishes
David
Hi David,
I don't have AD797 schematic, just simplified version from the datasheet, and I am not competent enough to put my fingers in the Scott's masterpiece.
By the way my complementary jfet preamp is not CFA, but non global negative feedback and uses a current conveyor as a gain stage. CFA version uses bjts and is symmetric complementary too.
BR Damir
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Old 20th April 2017, 08:38 AM   #9
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Quote:
Originally Posted by dadod View Post
I don't have AD797 schematic, just simplified version from the datasheet
The (almost) full schematic is AD797 guts

Quote:
By the way my complementary jfet preamp is not CFA...
Ok, but the circuit is the same as a CFA except you haven't connected any feedback

Best wishes
David
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Old 20th April 2017, 09:56 AM   #10
dadod is offline dadod  Croatia
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Quote:
Originally Posted by Dave Zan View Post
The (almost) full schematic is AD797 guts



Ok, but the circuit is the same as a CFA except you haven't connected any feedback

Best wishes
David
I would not call it CFA until Current Feedback is applied, it is just current conveyor. CFA could be used in very different circuit than this, but you know that.

I can't find compensating cap Cn in that schematic as in simplified one from the datasheet.
BR Damir
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