John Curl's Blowtorch preamplifier part II - Page 72 - diyAudio
Go Back   Home > Forums > Member Areas > The Lounge

The Lounge A place to talk about almost anything but politics and religion.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 10th August 2009, 06:32 PM   #711
syn08 is offline syn08  Canada
Account disabled at member's request
 
Join Date: Aug 2005
Location: Toronto
Quote:
Originally posted by PMA


Oversimplified textbook true. No real part behaves like this.

Quote:
Originally posted by PMA
This is much better. + channel length modulation, that depends on Vds.
Excellent, you must be my electron device physics PhD advisor in disguise

Seriously, before copy/pasting and declaring "much better" such, you'd better do your homework and make sure you understand when, where, for what devices and under which circumstances this formula holds.

Anyway, it does not include any channel modulation effect due to the longitudinal field (which is what counts an any device since the 60's) and is simply the result of a series expansion of the atan model that Scott mentioned. For all practical purposes, it isn't in any way shape or form much better than the classic parabolic and Lambda corrected Id=f(Vgs) for any devices currently available.

Here's a comparison between the two approaches.

And BTW, your beloved Spice you used to prove to the world the JFET matching benefits uses exactly the parabolic model with Lambda channel modulation
  Reply With Quote
Old 10th August 2009, 06:35 PM   #712
PMA is offline PMA  Europe
diyAudio Member
 
PMA's Avatar
 
Join Date: Apr 2002
Location: Prague
Thanks, I know the MC model (you can check my website). Keep cool. 有话好好说
__________________
Pavel Macura
http://web.telecom.cz/macura/audiopage.html
  Reply With Quote
Old 10th August 2009, 06:40 PM   #713
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Quote:
Originally posted by forr
Janneman
---This Perrot is that the guy aka Hephaistos?---

Hephaistos was Gérard Perrot's pseudo for the articles he published in french magazine "L'Audiophile". A great part of them were about the design of very linear transistorised circuits aimed to avoid thermal distorsion.

OT
That's what I thought. I have a lot of "Audiophile" articles by him (and Jean Hiraga), great articles, that mostly didn't get the exposure they earned, possibly because they were in French. But they were at least as good and astute as anything in the English press at the time.
/OT

jd
__________________
If you don't change your beliefs, your life will be like this forever. Is that good news? - W S Maugham
Check out Linear Audio!
  Reply With Quote
Old 10th August 2009, 07:18 PM   #714
diyAudio Member
 
john curl's Avatar
 
Join Date: Jul 2003
Location: berkeley ca
And people wonder why I just measure, rather than model!
  Reply With Quote
Old 10th August 2009, 10:31 PM   #715
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
Quote:
Originally posted by john curl
And people wonder why I just measure, rather than model!
With you there, John.

  Reply With Quote
Old 10th August 2009, 11:04 PM   #716
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
Quote:
Originally posted by Nelson Pass


With you there, John.

I even solder sometimes a real application before measuring.

__________________
The Devil is not so terrible as his math model!
  Reply With Quote
Old 11th August 2009, 03:15 AM   #717
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Hi Jan,
Quote:
One point he made was that if you use a servo to correct DC offset from a non-matched diff pair, you also base the pair off of the center point where the transfer function is as linear as it gets. Any off-centering (which the servo would do) would place the pair in a less linear region.
Yes. He outlined that both in his book and from some other material he had prepared (or had prepared). I found this to be comforting since I had been seeing this in a practical sense for many years. Do, then measure.

In fact, I had begun a practice of measuring distortion before and after the matching process. The measured performance was not that different, but then I was measuring total harmonic distortion only. The reason for this was that I had always matched transistors in the long tailed pair. I was taught that the hFE was the critical parameter, and it made sense to me that it would. After all, the diff pair's job is to compare the input signal and output signal, then to correct to reduce the error term (the difference). Complex math was never required for this. The concept is dead easy to understand.

Anyway, for a number of years, customers would come back, or call, asking what I had done to make the amplifier sound better. I was baffled for a long, long time. After all, I was merely following the directions in the service manuals and application notes that I still read today (current material, although I do refer back from time to time). I didn't occur to me that the beta matching of input pairs was as critical as it seems to be. That's why I started to examine what the effects of what I was doing were.

This never seemed to be questionable in any way. It wasn't until I began reading posts here that there was cause to re-examine what I had accepted for so many years. I must say that from the continuing tests I had been doing, the "match by beta" idea has only been re-enforced over the years. Another thing I've noticed is that vBE is a parameter that varies as temperature and process more than anything else. It seems to be that beta is the variable that needs to be nailed down.

Sorry, a very long winded way to say that the information Mr. Self has published does agree very well with what I have observed for so long.

Quote:
Intuitively I feel that the effect should not be very great, and there is still the global nfb to straighten things out, but the effect is there and can be measured on the ol function.
Actually, this is exactly the match is so critical. How does using nfb reduce the distortion produced in the diff pair if they are not matched? This is really an important point from what I understand.

The concept of negative feedback hinges on something we take for granted. We are assuming that the summing point is balanced and works as advertised. Well, if those two transistors are not matched, the differential pair does not properly extract the difference information. It will then introduce more distortion and fail to fully correct the existing error. As the mismatch becomes greater, the problems grow greater, but at an accelerated rate. This agrees with a chart I saw in his third edition book on page 81, figure 4.6. I am sure glad I bought his (excellent) book, it gave me something to refer to since I am unpublished. Actually, the first few lines at the beginning of chapter 4 illustrate this point very clearly. The first author in a long time that I've agreed with so strongly.

Now, my understanding may be at fault here. Please, feel free to comment.

-Chris
__________________
"Just because you can, doesn't mean you should" © my Wife
  Reply With Quote
Old 11th August 2009, 03:30 AM   #718
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Hi Joshua,
Quote:
What would you match for JFETs in complimentary differential pairs, other than IDSS?
Well, Pavel was quick and (correct) to answer that. Like many others, I don't have the ability to run curves on my components, so I spot check a few things. The pinch-off voltage and Idss are the two parameters I look at. If the job is really critical, then maybe a couple more points are measured. That's really a cross-check on my previous work. The transfer characteristic will line up pretty close as long as you can define two widely spaced points on the curves. Thank goodness for that!

When matching any transistor types, temperature will be the biggest challenge. Everything shifts as the temperature changes. That is why I match power devices by roughly controlling the temperature. For all signal devices, I match them in a jig I built that is a differential pair, with no feedback. I select the tail current and vDS and allow the pair to settle in. They are in physical contact and surrounded by a foam cover. I have mentioned this before, and I have posted the circuit a couple times as well. Basically, the parts are experiencing very similar conditions that they would see in actual use. The degree of matching is indicated by the voltage on their collectors (drains) across a pair of tightly matched resistors. The base (gate) resistors are also tightly matched. This setup is valid for pairs that are matched, not when the collector (drain) voltages differ. Since it is a match we want, no problem. As I have implied, the matching jig is both sensitive and works well for both BJT and FET type transistors.

-Chris
__________________
"Just because you can, doesn't mean you should" © my Wife
  Reply With Quote
Old 11th August 2009, 03:41 AM   #719
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Hi Bonsai,
Quote:
I am using 4 bipolar input devices on my amp and my source and speakers are quite good - I don't pick up the brittle sound you mention. Betas were matched on a meter to within 10%.
It's possible that your match is good enough, I can't say because this is your work.

I guess the big test for you would be to attempt a closer match and see if anything gets better. If there was anything that might mask the effect, you may not be able to tell. After all, your reference is usually the best thing you have heard so far.

One more comment, partly a joke. There are plenty of audiophiles around that think a Shure V15 MKIII sounds smooth. 'nough said right there.

Hi Andrew,
Quote:
Maybe we test differently.
That's possible. But you did mention you didn't have access to a THD meter? An HP 3586A, 3581A or 3580A would have worked well for you. To be honest with you, a lack of more advanced equipment can blind you to things simply because you are unable to measure them. This may explain why you don't see the things I am. The beta test on DVMs are not reliable, and the temperature will be your greatest 2nd variable that you can not really control. If the temperature can not be controlled, you must then test in a way that cancels temperature changes. You need to test them in pairs that share a common thermal environment.

-Chris
__________________
"Just because you can, doesn't mean you should" © my Wife
  Reply With Quote
Old 11th August 2009, 04:25 AM   #720
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Hi Andrew,
Quote:
I accept that I cannot hold a device to a set temperature.
Don't feel bad. I can't either. Since we can't change this, reduce your measurements sensitivity to temperature.

Quote:
I set up a long tail pair with the bases/gates directly connected.
I also directly connect the emitters/sources.
Here is where we differ. Only my emitters (sources) are connected directly together. I like to be able to measure base current in some way.

Quote:
Now add a pair of accurately matched collector/drain resistors.
Yes.
Quote:
Apply a Vbe/Vgs and measure the difference in collector current.
I use a current sink for the pair. The base will automatically draw the current it needs. vBE differences will certainly affect the outcome at this point in the test. The parts are tested in the sme manner that they are used in.
Quote:
This measurement method with Zero value for the External emitter/source resistor gives a very different result compared to Anatech's method.
Of course it will. They way I do it, each transistor is free to settle in where they "want" to be. That means they are also free to shift the balance of tail current. You have forced a situation where the bases (gates) are forced to the identical voltage and emitter (source) voltage.

Quote:
The zero Re is very intolerant of Vbe differences.
Yes, that is correct. That is also why the emitter (source) connection is common to both devices.

Hi Jan,
Quote:
One point he made was that if you use a servo to correct DC offset from a non-matched diff pair, you also base the pair off of the center point where the transfer function is as linear as it gets
Exactly! Thats why many designs really don't need a servo. The DC offset can be brought less than 10 mV in practice without any heroics. I guess someone out there decided that DC offset must be less than 2 mV.

-Chris
__________________
"Just because you can, doesn't mean you should" © my Wife
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



New To Site? Need Help?

All times are GMT. The time now is 10:48 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2