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Old 9th February 2006, 03:34 PM   #1
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Default The Cross Quad diff

Hi,

after a small chat in Suzy thread, ETI 5000 MOSFET Power amp, I posted a link to a schematic from Dr Bora on his Delta amplifier which is quite unusual at least in that sense it uses in audio gear a very seldom seen "Cross Quad" diff pair at the IP.

That led me to start this thread as I wanted to know if other have seen and/or used it, and bring it out in the light.

At a first look it cheats one's eyes as it doesn't behave as "expected" with a normal diff pair, it has actually an inverted output in comparison to the "normal" diff pair.

Analog Devices shows an exampel of the cross quad in their datasheet for
MAT04 which starts on page 8 with a shematic seen on p.9, though with an additional diff pair on top which in this case acts as the input.

Dr Bora's Delta amplifier schematic can be seen here.

Your thought's and experiences?

Cheers Michael
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Old 9th February 2006, 03:53 PM   #2
ilimzn is offline ilimzn  Croatia
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Default Re: The Cross Quad diff

Quote:
Originally posted by Ultima Thule
At a first look it cheats one's eyes as it doesn't behave as "expected" with a normal diff pair, it has actually an inverted output in comparison to the "normal" diff pair.
Not as far as I can see in the Delta schematic...
It should also work with MOSFETs, but most simulators have problems with it, sop i guess it's back to the old 'build it and see' ways
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Old 9th February 2006, 10:32 PM   #3
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Ask directly dr Jagodic whether he built it and if so, how does (n't) it work.
I'm interested....

About croos-quad... I have not used it, I don't see many advanteges, but one huge disadvantage- possible saturation of some of input bjts at clipping/slew overload.
Look to Hawkford's site for more info.

regards
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Old 9th February 2006, 11:47 PM   #4
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Darkfenriz,

you should now the answer yourself!

I should point out that it's my belief too that the Delta schematic is drawn wrong, you made the right correction Darkfenriz.

Did you noticed that Ilmzin?

And yes, I have read the Hawksford paper.

Cheers Michael
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Old 10th February 2006, 12:11 AM   #5
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The strange thing is... I started to doubt.... and simmed just as author drawn it and it simmed (1kHz) flawlessly with both 'positive' and 'negative' feedback...
unless fed with a sqare, than the original design showed oscillations at some 8MHz
Great mistery to me
I think this can have something to do with saturation of cross-quad and changed polarity of inputs (TL071 style), but I don't know.
The only 'rational' explaination is the simulator gets fooled by kind of classD work of some stages with high carrier frequency... and it is not visible in sim.
So this actually could work, that's why I would be interested.
very interested

regards
Adam
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Old 10th February 2006, 12:25 AM   #6
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Ok,

I see...

Because when I look at the cross quad I do understand why it get's inverted at it's output, hmm.. does it behave differently for small signals compared to large signals?

Adam, did you sim just the cross quad alone, it would be best to just isolate it for now and study it, if I find the time I will build it tomorrow Friday.

Thanks for your input,

Cheers Michael
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Old 10th February 2006, 12:41 AM   #7
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Quote:
Originally posted by Ultima Thule
does it behave differently for small signals compared to large signals?
Exactly
horisontal is difference voltage and the curve is collector current of one output 'legs'.
So sometimes it can work provided never switched to different 'monotonics' part of the curve, which can happen only in noiseless/clipless/TIMless simulators.
I guess it deserves further investigation.
cheers
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Old 10th February 2006, 01:22 AM   #8
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I simulated the cross quad against a conventional LTP at the same current, collector load etc.

Phase is reversed
Gain is about 30dB greater
Gain Bandwidth Product is the same
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Old 10th February 2006, 02:57 AM   #9
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It needs emitter degeneration to stop it oscillating on sim. I just added some 10Rs and now the bandwidth is improved, but becomes 2nd order at roll off. Gain is still much higher than the simple LTP
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Old 10th February 2006, 05:38 AM   #10
Tim__x is offline Tim__x  Canada
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How does it work? My eye sees a positive feeback loop around Q3 and Q4, actually I see positive feedback into both the base and emitter of both Q3 and Q4.

I tried simming it, it works fine in small signal analysis, but in .tran it quickly latches up and acts like a normal LTP.
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