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Old 6th September 2003, 04:44 PM   #1
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Cool my take at (true) X input stage

hey all,
let me start by thanking Petter, on whose work i built.
Nelson Pass once stated in the old X100 backengieneered thread that the X1000 uses cascoded input devices "to keep the power dissipation down"..
But i think the true reason is that you need the additional cascode to be able to truly make an X circuit. The distinguishing feature, from my point of view, is that you take feedback "from the other side". When you have only the folded cascode, as in Petters and Uli's amps, you have to feed back from the same side - so in my view these are not X amps - where's the X?
(the Aleph X has it, no need to get excited)
the "magic" resistor that couples the diff amps tails has nothing to do with it, it is just a current feedback mechanism - it is not new or anything, it's in my circuit design book actually...

Please take a look at the attached schematic.
there are a few extra resistors to make the sim converge..(R12-R15) + comp. caps 1pF each.
the X is below the source, in the middle...
I'm looking forward to your comments.
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File Type: gif keyne-xinput.gif (11.4 KB, 986 views)
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Old 6th September 2003, 05:33 PM   #2
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Depending on the polarity of the amplification which follows
the differential input pair, you take the feedback from either side.

Odd numbers of Comon-Source gain stages (including the diff
pair as a gain stage) use the same side, and on even they
use the opposite side for the source of feedback.

The X circuit does not require cascoding, but certainly can be
made to benefit from it, just like any other amplifier.
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Old 6th September 2003, 05:40 PM   #3
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Hi Keyne

I don't think I agree, not yet atleast ...

The Q5 and Q6 do nothing for the polarity of the voltage or current signal involved., so I don't see how they can make a difference for the feedback 'take-off points' ...

The upper nodes of Q2 and Q3 don't feature (much) voltage swing .. they are allready cascoded, allthough with a folded a additional cascode would not make much of a difference for linearity or bandwith...

I think the additional cascodes Q5 and Q6 serve to spread the dissipation ... putting less heat stress on each of the MOSFETS..

I'll try some simmulations my self.... let's hear it from the others..

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Old 6th September 2003, 05:45 PM   #4
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Mr Pass,
thanks for your comment, i see my error now!

I've been fooled by my simulation...

Mr. Pass, so the distinguishing feature is not the "X" in the schematic, but the distortion cancellation mechanism at work, right?

Thijs, your points are correct.
I still think the additional cascode is good, though. decreases voltage over the diff pair significantly.

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Old 6th September 2003, 09:09 PM   #5
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"X" isn't cascoding, it's just a strategy for feedback. The one requirement is for the circuit to be balanced...or at least build off of a differential. The connection (of the feedback signal) between cathodes/emitters/sources is the ticket. All else is just icing on the cake.
Cascoding is a nifty addition, but isn't essential, per se. (It's just in the patent to strike fear into the hearts of heathens and unbelievers...)

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Old 6th September 2003, 11:29 PM   #6
MikeW is offline MikeW  United States
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Nice to see a post from you Grey.
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Old 7th September 2003, 04:27 AM   #7
Apogee is offline Apogee  United States
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Welcome back Grey!

Also very glad to see a post from you...

Please don't let the screaming monkey(s) run you off again...

Some of us out here appreciate your contributions very much... It also makes this place a bit more fun!

Hope all is well...

"If I had my life to live over again, I would have made a rule to read some poetry and listen to some music at least once a week." - Charles Darwin
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Old 7th September 2003, 07:11 AM   #8
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happy to see you again!

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Old 7th September 2003, 07:16 AM   #9
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Wonderful to read your posts,

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Old 7th September 2003, 06:44 PM   #10
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Grey is back!

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