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Old 15th June 2009, 02:12 AM   #19561
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Nice to see you here, Nelson.
 
Old 15th June 2009, 11:17 AM   #19562
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Default tanh stuff

Quote:
Originally posted by scott wurcer
As Wavebourn said there is more than one way to skin a cat. Here is the AD8099 input linearizer (which does NOT compromise the noise unlike the tanh technique), US6963244. You would like Nathan a lot, he was at the Bongo dinner too . You probably need to read the patent for a full understanding. This was one of two techniques in house and his won.
Hi Scott,

I fiddled around with this idea and results were very promising. By adding cascodes, the tail modulation circuit gets really simple.
Regrettably one will need an exact ratio of emitter areas (~4.1), not easily achieved with discrete components.

BTW, why was this (rather obvious) variant not covered by the patent?


Cheers,
Edmond.
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File Type: png tanh1.png (12.7 KB, 595 views)
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Old 15th June 2009, 12:45 PM   #19563
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Default Re: tanh stuff

Quote:
Originally posted by Edmond Stuart


BTW, why was this (rather obvious) variant not covered by the patent?

Cheers,
Edmond.
Maybe headroom? I would have to look. There is already a new variant to handle this issue. It's becoming hard to sell amplifiers that don't go to one rail on the input and both on the output.

I still think this circuit would be more amenable to just "piling on" to make the 4X diodes. You can still get good results with 4 to one and here you just have two diodes to make, in fact I think getting the right total delta-Vbe across both is good enough since this is again a translinear circuit. The 4.1 IIRC is the same ideal point as the multi-tanh.

I'm checking into the third technique.
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Old 15th June 2009, 03:34 PM   #19564
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Default Re: Re: tanh stuff

Quote:
Originally posted by scott wurcer
Maybe headroom? I would have to look. There is already a new variant to handle this issue. It's becoming hard to sell amplifiers that don't go to one rail on the input and both on the output.
Hi Scott,

Does this mean that cacoding is an obsolete technique by now?


Quote:
I still think this circuit would be more amenable to just "piling on" to make the 4X diodes. You can still get good results with 4 to one and here you just have two diodes to make, in fact I think getting the right total delta-Vbe across both is good enough since this is again a translinear circuit.
Agreed, a ratio of 4:1 is good enough (if that's what you mean), but a THAT300 chip for example, only contains 4 match trannies, while we need 4+2=6 of them.

Quote:
The 4.1 IIRC is the same ideal point as the multi-tanh.
Of course the same ideal ratio applies, because it's essentially the same circuit. It only differs in that the modulation current (through Q5 & Q6) is directly subtracted from the tail currents (via the common emitters of the input stages), instead of via additional current mirrors (Q11,Q12,Q15 & Q16 as in fig.5 of the patent)
Also notice that in my variant, Vcb of all input and modulating trannies is kept at the same level (i.e. 0V)


Quote:
I'm checking into the third technique.
Yes please.

Cheers,
Edmond.
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Old 15th June 2009, 04:05 PM   #19565
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Default Re: Re: Re: tanh stuff

Quote:
Originally posted by Edmond Stuart

Hi Scott,

Does this mean that cacoding is an obsolete technique by now?

No, just the brute force techniques like those that included another level just to accomidate Ib comp transistors are not that useful anymore. Two volts from each rail does not leave much on a 5V supply .

I was wondering this morning if combining Vbe's and a thermistor of just the right TC might make one of these work in discrete form? Sort of a TC compensated area cheater. Do you know what I am refering to? Take a diode connected transistor and put a small resistor from base to collector and sample the diode voltage at the collector. I have not gone through the math on this technique. Your circuit is an excellent variant BTW it should make clear the operation to everyone.
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Old 15th June 2009, 04:21 PM   #19566
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Default cascade

Hmm... if a cascode is unacceptable in these modern times, how to compensate for the Early and nonlinear Cob effect?
Is this old geezer missing something?

edit: apparently my response was too fast. See the next one.
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Old 15th June 2009, 04:36 PM   #19567
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Default Re: Re: Re: Re: tanh stuff

Quote:
Originally posted by scott wurcer
No, just the brute force techniques like those that included another level just to accomidate Ib comp transistors are not that useful anymore. Two volts from each rail does not leave much on a 5V supply .

I was wondering this morning if combining Vbe's and a thermistor of just the right TC might make one of these work in discrete form? Sort of a TC compensated area cheater. Do you know what I am refering to? Take a diode connected transistor and put a small resistor from base to collector and sample the diode voltage at the collector. I have not gone through the math on this technique. Your circuit is an excellent variant BTW it should make clear the operation to everyone.
I was tinkering with that idea too, but it's not that simple. If the cuircuit also depends on resistor values, much chance that it also depends on the value of the current sources (I1 & I2), making things even more complex.

Below an example of a DIY version based on two THAT chips.
As already pointed out, the optimal value of R1 depends on Tamb AND I1 & I2!

Cheers,
Edmond.

edit: >Your circuit is an excellent variant BTW it should make clear the operation to everyone.
Thank you!
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File Type: png tanh3.png (15.5 KB, 536 views)
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Old 15th June 2009, 05:58 PM   #19568
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Edmond,
From this tube hounds eyes, I can't see how that circuit can work. The output seems to be only connected to a couple of CCS's, which in turn are only connected to the rails.

Electronic telepathy perhaps?

Regards, Allen

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Old 15th June 2009, 06:02 PM   #19569
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Quote:
The output seems to be only connected to a couple of CCS's
These are CCCS
 
Old 15th June 2009, 06:06 PM   #19570
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And what, pray, is a CCCS?

Regards, Allen

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