Discrete Opamp Open Design - Page 177 - diyAudio
Go Back   Home > Forums > Source & Line > Analog Line Level

Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

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 16th October 2012, 09:43 PM   #1761
bcarso is offline bcarso  United States
diyAudio Member
 
Join Date: May 2005
Location: Canoga Park, California
Default better stats on the BF862

For statistics 30 samples is a sort of magic number for enhanced confidence levels. So I added another 11 pieces to the set and have the following:

Mean value for Idss at Vds ~3.846V: 15.368mA
Standard deviation in Idss: 1.787mA
Mean value of Vgs for Id = 100uA, Vds = 3.999V: -0.6240
Standard deviation in Vgs as above: 50.23mV

As before, I subtract the voltage drop in the 10.0 ohm drain resistor, hence the average Vds is as shown but varies a bit. I could get fancy and make the drain feed a virtual ground at precisely 4V I suppose.

Observations made more systematically for the last 10 units involved noted the initial Idss on power-up and then the value after 30s. The recorded value for the crunching was the 30s value, but the difference was observed, and from it I can say that, at least for this batch, zero tempco occurs around 15.5 to 15.8mA Idss, and for a close-to-threshold -630mV Vgs. One could conjecture that this was the designer's target, as those values are not far from the mean values for the 30 pieces.

Outliers: One part had Idss = 10.406mA, 100uA Ids at Vgs = -483mV. The highest Idss was 18.275mA, corresponding Vgs of -701.6mV.

It is difficult for me to estimate the thermal resistance from the chip to the gate contact in the test fixture, which is a gold-plated finger a bit wider than the lead. And I don't always center it perfectly. However, a swag based on the device's maximum rating would estimate the chip temp change at Idss from ambient as about an average of 40 C, in which case some of the parts would have about a 6uV/degree C tempco. I doubt that matching in such cases would be of much importance for some applications, and one would be better off just using a part single-ended. However with resistance in the sources everything changes.
  Reply With Quote
Old 16th October 2012, 10:48 PM   #1762
fas42 is offline fas42  Australia
diyAudio Member
 
fas42's Avatar
 
Join Date: Jun 2012
Location: NSW, Australia
Blog Entries: 10
Quote:
Originally Posted by bcarso View Post
Mean value for Idss at Vds ~3.846V: 15.368mA
Standard deviation in Idss: 1.787mA
Mean value of Vgs for Id = 100uA, Vds = 3.999V: -0.6240
Standard deviation in Vgs as above: 50.23mV
You have to be impressed at times by the quality of manufacturer's specs ... and modelling. Or maybe it's a sheer fluke ...

I've been playing a bit with the model of the BF862, adjusting the parameters so to get a high correlation with the NXP data sheet. They show a curve of the typical Id vs Vgs, as well as max and min examples. Now, I've got an instance of a BF862 that matches that typical curve, so I plugged in a Vds of 3.8V and ran a parameter sweep: Idss = 15.4mA ...

Queue X files music ...

Frank
  Reply With Quote
Old 16th October 2012, 11:49 PM   #1763
bcarso is offline bcarso  United States
diyAudio Member
 
Join Date: May 2005
Location: Canoga Park, California
Quote:
Originally Posted by fas42 View Post
You have to be impressed at times by the quality of manufacturer's specs ... and modelling. Or maybe it's a sheer fluke ...

I've been playing a bit with the model of the BF862, adjusting the parameters so to get a high correlation with the NXP data sheet. They show a curve of the typical Id vs Vgs, as well as max and min examples. Now, I've got an instance of a BF862 that matches that typical curve, so I plugged in a Vds of 3.8V and ran a parameter sweep: Idss = 15.4mA ...

Queue X files music ...

Frank
That's very cool

Next step: test a few more and parallel them in the manner I alluded to some posts back, which should be best both for heat dissipation and for minimal parasitics. I don't have test equipment handy for direct observation of energy at ~500MHz (although I do have a Tek 485 in storage that could be brought to bear), but a little literal hand-waving while observing d.c. bias is usually sufficient.

It's not that I have any objections to inductors in the gate leads, but rather would find it fun if the issues of dissipation and RF oscillations can be dealt with simultaneously. And as I suggested in my LTE to Linear Audio, maybe for some of these extravagant designs it's not too far-fetched to consider servoed thermoelectric cooling, rather than simply temperature compensation. As long as one doesn't flirt with the dew point it would seem a decent approach.
  Reply With Quote
Old 17th October 2012, 01:24 AM   #1764
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
My BF862 models use defaults for all parameters over temperature, not sure the sim results are that great for drift. As we know there are two sources of drift in a JFET so unlike a bi-polars the drift is not zero when the offset is zero. OTOH there is a combination of two trims in the right circuit where they can both be zeroed.

We should actually build some and see what we need to do. Remember a high gain low noise version could use LSK389's without penalty.

BTW my tube purchased years ago is similar, right on process center.
__________________
Silence is so accurate.
  Reply With Quote
Old 17th October 2012, 02:30 AM   #1765
RNMarsh is offline RNMarsh  United States
diyAudio Member
 
RNMarsh's Avatar
 
Join Date: May 2012
Location: 2457 Cascade Trail; Cool, CA. 95614
What happened to just physically coupling the diff pair with a little grease between them.... a small clip/mass coupling them together? Maybe harder to do with smd, though. A dual device should be fine for drift issues... then just trim resistors to get it to zero and stable.

Last edited by RNMarsh; 17th October 2012 at 02:39 AM.
  Reply With Quote
Old 17th October 2012, 02:35 AM   #1766
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
Quote:
Originally Posted by RNMarsh View Post
What happened to just physically coupling the two diff pair with a little grease between them.... a small clip/mass coupling them together?
I still think this is fine, let's try a few and see rather than go over all the things that can go wrong.
__________________
Silence is so accurate.
  Reply With Quote
Old 17th October 2012, 02:41 AM   #1767
EUVL is offline EUVL  Europe
diyAudio Member
 
Join Date: Oct 2003
Pass Discrete Opamp in DIP-8 Package
All devices glued to the sink with Arctic Silver adhesive.




Patrick
__________________
xen-audio.com
  Reply With Quote
Old 17th October 2012, 02:56 AM   #1768
diyAudio Member
 
Join Date: Nov 2011
Location: Cooktown, Oz
Quote:
Originally Posted by scott wurcer View Post
Out of curiosity which version the 2SXXXX or the BXXX, one runs at 500ua and the other at 1ma, I don't see 3dB noise figure from 5 Ohms at those currents?
I have seen some folks use power transistors with 1 Ohm rbb to get there even then at more current.
Scott, I'm not sure what your question is but I got rbb about 5R from Hitachi a108x & c254x circa 1980.

Baxandall and myself looked at a lot of medium power devices in da old days but I stopped looking once I found the Hitachis. The power devices had high 1/f noise especially their current noise and none were as good as the Hitachis.

You can still get c254x, Toshiba c3329 & Rohm d786 in qtys for small production but their pnp complements are truly Unobtainium.

BTW, Low Noise Design Schematics shows rbb = 1R7 for c2547 which I don't believe. If all his noise measurements are as wonky, he is even further away from my little circuit. Also his favourite LN BJTs are the same as mine so he doesn't get around the Unobtainium problem at all.

[edit]Scott, just realised you've discovered my secret identity. The 0.28nV/rtHz version runs 3mA total with 2 of the Hitachi devices. More current doesn't help which tends to confirm the rbb. The 2 'commercial' versions didn't use more current cos there weren't any MC amps at that time that were quieter with real MC cartridges .. even with my non-optimal noise matching. Description is in README.doc[/edit]

Last edited by kgrlee; 17th October 2012 at 03:25 AM.
  Reply With Quote
Old 17th October 2012, 03:07 AM   #1769
bcarso is offline bcarso  United States
diyAudio Member
 
Join Date: May 2005
Location: Canoga Park, California
Quote:
Originally Posted by kgrlee View Post
You can still get c254x, Toshiba c3329 & Rohm d786 in qtys for small production but their pnp complements are truly Unobtainium.
Or very costly. Just checked eBay for the 2SA1316. Yow! Someone has some. (As do I [chortles]).
  Reply With Quote
Old 17th October 2012, 01:43 PM   #1770
diyAudio Member
 
grhughes's Avatar
 
Join Date: Mar 2005
Location: Cubicle Sweet Cubicle, Springfield, Missouri, USA
In SMT using BF862, wouldn't it be best to thermally couple the two njfets input diff amp with a flat aluminum bar and epoxy it to the two top flat surfaces. Do the current mirrors need thermal sinks? Since the input stages are cascoded, how far can we push the + and - rails towards 24v. I think simple coupling with 6061T6 aluminum stock would be something DIYs could do rather than potting. And trim pots can do the rest of balancing. Potting is messy! Are we are gona have a group buy of matched transistors, or smt circuit boards? Thanks, Ray
__________________
" "The fool doth think he is wise, but the wise man knows himself to be nothing but a fool." Will Shakespeare
  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


Similar Threads
Thread Thread Starter Forum Replies Last Post
Discrete OPAMP audio-gd Vendor's Bazaar 27 20th September 2012 04:02 PM
discrete opamp help blackpowderaudio Parts 0 16th December 2009 03:46 PM
THAT transistor headphone amp (250ma discrete opamp) design sanity check. Russ White Headphone Systems 19 13th December 2007 12:52 PM


New To Site? Need Help?

All times are GMT. The time now is 08:43 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