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Current-Source-Driven-power-transistors-and-mitigating-cross-conduction-distortion
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Old 30th March 2017, 09:19 AM   #1
IanHegglun is offline IanHegglun  Australia
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Default Current-Source-Driven-power-transistors-and-mitigating-cross-conduction-distortion

This thread is to hopefully find ways to reduce distortion due to high frequency cross-conduction in current source driven power transistor Class-AB output stages.

An example of this problem is Wim de Jagger's "New Class-B" amplifier published in Electronics World December 1999 p982–7 A NEW CLASS-AB DESIGN

This new topology creates distortion from cross-conduction in the high end of the audio range. See Wim's Figure 11 (attached or view on my Gdrive - see my member info for link). Distortion is 0.1% at 20kHz. It rises steadily from 0.01% at 1kHz. Most of this distortion is from cross-conduction which is due to transistor charge storage because no base-emitter resistors are used and the bases of the power transistors are current driven. This unconventional approach solves the seemingly unsolvable thermal problems of Class-AB biasing stability, but instead we get more distortion from cross-conduction and more waste heat from cross-conduction.

Another example is described in Linear Audio Volume 13, "Current driven Output-stage class AB power amplifier" https://linearaudio.net/article-detail/2287. A different topology to Wim's but one that also drives the bases of the power transistors with current sources. There doesn't seem to be a colloquial term for this new type of Class-AB (see thread). I'll call it 'CSD' because it uses Current Source Driven power transistors, until someone comes up with a better one?

The Linear Audio 'CSD' amp gave 0.1% at 1kHz. Not good spec's for selling but to my ears it sounded as good as the Cube-law Class-A design with 30 times lower THD. Still, it would be nice if ways can be found to easily reduce the THD by removing most of the cross conduction; and that's the main aim for this thread.

Feel free to consider related things, like building a CSD amp, which can be done using a Bimodal PCB which is a Cube-law amp (Linear Audio Vol.8) but with CSD bipolar power transistors instead of lateral MOSFET's.

I have placed some LTspice simulation files on my Gdrive for anyone to use. The PDF 'BiModal-CSD+SqA-nsb' compares two 'nsb' methods. By adding a small square-law driver component to the linear driver current adds on 50% to the idle current but adds a true Class-A component extending to higher power levels and seems to mitigate distortion from cross-conduction, but it doesn't do anything to reduce cross-conduction at high audio frequencies.

Simply adding base-emitter resistors for fast base charge removal wipes out current drive and brings back the thermal biasing stability to the standard output stage. Adding a current source instead of base resistors creates a sharp turn on/off at the start of the crossover region and this sounds worse than cross conduction distortion because it generates high order distortion. And adding a constant current from the drivers, like a non-switching class-B current, you are simply cancelling two constant current sources, so nothing gained. One idea that might help is a current mirror in a feedback loops with the power transistors. A jig is provided for download on my Gdrive as well. It can reduce cross-conduction dissipation but needs heavier frequency compensation, so no a lot of distortion reduction at the high frequency end.

These are a few starters. If you are bored by this technical stuff why not consider making a CSD amp, or a Cube-law amp. I'm avaiable to help DIYer's as much as I can. BTW these amps are not suitable for first time builders.

Ian Hegglun
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Old 30th March 2017, 11:32 AM   #2
jan.didden is online now jan.didden  Europe
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Ian, thanks for posting this design here. There's a good chance someone comes up with a nice solution that has been overlooked until now.

I will see that I can make the Vol 8 and Vol 13 articles available as a single PDF, need to check with my prepress guy.

Jan
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Old 30th March 2017, 12:56 PM   #3
Dave Zan is offline Dave Zan  Australia
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Quote:
Originally Posted by IanHegglun View Post
This thread is to hopefully find ways to reduce distortion...
Hi Ian
I started a thread on current drive just a couple of months back, it's >Here<.
Jan Didden foreshadowed your article and I am very pleased to see you here in the forum, I look forward to more discussion.

Best wishes
David
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Old 30th March 2017, 09:22 PM   #4
padamiecki is offline padamiecki  Poland
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Hi
I am surprised of how the telepatic way People do think...
I am subscribed and looking to the posts, BTW LA13 printout is probably on the way to me!
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Old 31st March 2017, 08:38 AM   #5
IanHegglun is offline IanHegglun  Australia
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Quote:
Originally Posted by Dave Zan View Post
Hi Ian
I started a thread on current drive just a couple of months back, it's >Here<.
Sorry, I missed seeing that.

Dave, your current-drive thread is a good one for general discussion on different ways to use current drive in transistor output stage. My thread is specific to reducing cross-conduction distortion in my output stage descried in Linear Audio Vol.13 ...

And my latest circuit can be viewed in my CDS Addendum (downloadable from my Gdrive), and you can run the Circuit in LTspice (download the BiModal Zip file).

In my opening post I mentioned making Linear Audio amps. But now I just noticed the existing Linear Audio Cube-amp thread is still active (thanks Jan Didden), so maybe it would be better to recommend that talk about making the Linear Audio amps take place in the existing Cube-amp thread.

That way we can concentrate on ways to get lower distortion from my output stage descried in Linear Audio Vol.13.

How's that sit to you?

And thanks for the welcome to the diyAudio community.

Cheers, Ian Hegglun
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Old 31st March 2017, 10:41 AM   #6
kasey197 is offline kasey197
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Current-Source-Driven-power-transistors-and-mitigating-cross-conduction-distortion
Ian welcome ! I have been reading your stuff for years and it's great to see you here
Ive learnt a lot from your work and look forward to learning more )
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Old 31st March 2017, 02:54 PM   #7
FdW is offline FdW  Netherlands
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Current-Source-Driven-power-transistors-and-mitigating-cross-conduction-distortion
Hi Ian

Thanks for dropping in, as I am really interested in current driven current sources to be used as power amplifier, I will follow this thread closely.

The second item of interest in your design is the way you implement soft clipping, as I am a strong belever of soft clipping I use one in my amplifier also.
The one I use is implemented with MosFet's but implemented differently, attached is a schema that I use. The MosFet's shown can be selected for exact 2VRMS or (when you do not like to discard most) in a range of 1.9...2.3VRMS (this gives about 50% keepers).

Best regards,
Frans.
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Old 31st March 2017, 03:16 PM   #8
Mooly is offline Mooly  United Kingdom
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Current-Source-Driven-power-transistors-and-mitigating-cross-conduction-distortion
I remember Wim de Jager's article from almost 20 yrs ago and remember thinking what an unusual design it was, more reminiscent of the internal architecture of an IC than a workable power amp.

Curious, I just simulated the original and find I can not meet the original specs. In fact distortion is pretty obvious in the sim without even looking at FFT's or number crunching. It also seems very transistor (model) dependent.

I wonder if anyone ever built this design and is so what they thought of it.
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Old 31st March 2017, 03:24 PM   #9
Elvee is online now Elvee  Belgium
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Some ideas could probably be lifted from this topology (with suitable adaptations, of course):
http://www.diyaudio.com/forums/solid...ml#post4954495
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Old 31st March 2017, 07:09 PM   #10
jan.didden is online now jan.didden  Europe
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Quote:
Originally Posted by Mooly View Post
I remember Wim de Jager's article from almost 20 yrs ago and remember thinking what an unusual design it was, more reminiscent of the internal architecture of an IC than a workable power amp.

Curious, I just simulated the original and find I can not meet the original specs. In fact distortion is pretty obvious in the sim without even looking at FFT's or number crunching. It also seems very transistor (model) dependent.

I wonder if anyone ever built this design and is so what they thought of it.
This delay in switching off is what causes the cross-conduction that Ian referred to. I believe that's what he is looking to fix.

Jan
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