How to find Spice models of bjts - Page 3 - diyAudio
Go Back   Home > Forums > Design & Build > Software Tools

Software Tools SPICE, PCB CAD, speaker design and measurement software, calculators

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 3rd July 2014, 07:34 AM   #21
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
The models we use in LT are (as I understand it) basic "1st order" models and don't represent fully the way a real device will behave. For example a real device with an AC signal across it will experience changes to junction capacitance, changes to gain and so on. Those parameters (again, as I understand it) aren't reflected in the models.

If you look at the data sheet for an LM4562 opamp and look at the method of testing distortion you will see that it says it is below current measurable limits. To get the real figure needs a simple trick... look at the data sheet to see how its done. (The same trick for the simple one transistor circuits won't work.) What I'm getting at is that in LT, its easy to get a circuit to perform "better" than the best currently available... so something 'aint right somewhere

What LT can do when used at this level is to guide you so that you can see what direction changes are heading, what the effect of certain changes are and so on but the reality won't exactly match the simulation. If it did then we would all be making world beating amplifier designs.
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
  Reply With Quote
Old 3rd July 2014, 08:32 AM   #22
diyAudio Member
 
Join Date: Sep 2009
Location: Stavanger (NORWAY)
Hi and thanks a lot again for the very valuable reply
I understand that ... i dont understand
I mean ... as usual i am trivializing the issue

But ...
1) can we say that some parts have better performance of others for some application ? like to give a gain voltage of 2 to the incoming signal ?
2) can we say that taken a component its performance varies a lot in function of the working conditions ?

I have one big problem ... to understand the optimum working point for a bjt from the data sheet of the part , given that this point do exist of course
It is very clear from the sim that the performance change a lot with Vce and so on ...
I think that this point should be easy to spot in a data sheet
What i have to look for ? in which graph this point can be spot ?
Like working conditions that give max linearity to the device (i am guessing here)
I think i have to read to book first for sure ... i am sounding much like a neanderthal man, electronics wise.
Kind regards, gino
  Reply With Quote
Old 3rd July 2014, 11:15 AM   #23
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
I haven't got all the answers, that's for sure.

Yes, some parts do perform better in certain applications, some will reliably show that improvement in LT and others will not.

When you increase the supply, the change in voltage across the transistor relative to the DC conditions becomes small and that makes a big difference to performance.

Two important characteristics are "early effect" and the "Cob" (collector output impedance).
Attached Files
File Type: pdf Lec_4_BJTBias1_08.pdf (230.5 KB, 13 views)
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
  Reply With Quote
Old 3rd July 2014, 02:52 PM   #24
diyAudio Member
 
Join Date: Sep 2009
Location: Stavanger (NORWAY)
Quote:
Originally Posted by Mooly View Post
I haven't got all the answers, that's for sure.
Yes, some parts do perform better in certain applications, some will reliably show that improvement in LT and others will not.
When you increase the supply, the change in voltage across the transistor relative to the DC conditions becomes small and that makes a big difference to performance
Hi and thanks and yes this is what i am seeing
Problem is that i find much more easier to read an op-amp data sheet than a bjt data sheet ... in the op-amp most of the graph are very clear

Quote:
Two important characteristics are "early effect" and the "Cob" (collector output impedance)
This is very difficulf for me now. I have to leave it for the future after reading the book.
Thanks a lot for the very interesting paper attached.
Unfortunately the only thing clear to me is the conclusion

Quote:
Biasing a BJT poses large stability problems, since its characteristics are highly sensitive to temperature and since its electrical properties (principally beta) vary widely from one device to another!
There is more than enough to get worried ... but i will read about this
Kind regards, gino
  Reply With Quote
Old 3rd July 2014, 03:03 PM   #25
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Remember that good circuit design "designs out" the effect that temperature and so on will have on a circuit. That is why a simple circuit like the Radford can work as well with a transistor with a gain of 50 or 500. It doesn't matter, it doesn't alter the AC gain of the circuit or the alter DC operating conditions.
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
  Reply With Quote
Old 3rd July 2014, 03:42 PM   #26
diyAudio Member
 
Join Date: Sep 2009
Location: Stavanger (NORWAY)
Quote:
Originally Posted by Mooly View Post
Remember that good circuit design "designs out" the effect that temperature and so on will have on a circuit.
That is why a simple circuit like the Radford can work as well with a transistor with a gain of 50 or 500.
It doesn't matter, it doesn't alter the AC gain of the circuit or the alter DC operating conditions
Thanks for the advice. Let's say that a very basic strategy to overcome the temperature effect may be is to use, as i intend to do, more powerful transistors well withing their SOA ... much well within actually. And maybe even heavily heath-sunk ?
Like the 2SC2238B for instance.
http://www.jmnic.com/pdf/2sc/2SC2238A.pdf
Then if one will be not enough i will pass at two ... but no more.
Or two or nothing .... when i look inside a line preamp and i see even more than 20 active devices per channel ... i wonder if this is really the right approach.
My simple mind would suggest me to use the very few devices in their optimum working conditions, conditions that at the moment i have no clue which they are but i have the feeling that can be seen in a dataheet somewhere.
Back to play with sims.
Kind regards, gino
  Reply With Quote
Old 3rd July 2014, 06:07 PM   #27
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Optimum conditions depend on your own specifications and intended use.

If you want a single transistor preamp (and the downside to that is that it inverts the signal) then you need to consider whether it can drive the intended load... and to make sure it can means you have to decide on the operating point to achieve that. Its no good saying you get optimal noise performance for example at xyz bias when that level of bias won't even drive a damp piece of string. Or conversely, if the optimum point is with it drawing a few hundred milliamps, then that too would be a compromise too far.

There are no rights and wrongs... the whole design has to be considered as a whole.
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
  Reply With Quote
Old 3rd July 2014, 06:30 PM   #28
diyAudio Member
 
Join Date: Sep 2009
Location: Stavanger (NORWAY)
Quote:
Originally Posted by Mooly View Post
Optimum conditions depend on your own specifications and intended use. If you want a single transistor preamp (and the downside to that is that it inverts the signal) then you need to consider whether it can drive the intended load..
Hi and thanks again
Just to explain i would like to make a line preamp like the single mosfet Bride of Zen but with a power bjt
I do not know if it is possible
the load should be a tipical solid state power amp ... like a 20k load (maybe with some capacitance) with 2V of sensibility
Sources will be only digital or a phono stage separated ...
If this is not possible i will stick to the BOZ that i quite like ... it has a nice dimension in sound, it is not flat at all
I have already some boards for that
But BJTs are more brutal ... i have in mind a 20W to220 from Toshiba or similar
That i can use at 60V let's say
But i have no model of those

Quote:
and to make sure it can means you have to decide on the operating point to achieve that.
Its no good saying you get optimal noise performance for example at xyz bias when that level of bias won't even drive a damp piece of string.
Or conversely, if the optimum point is with it drawing a few hundred milliamps, then that too would be a compromise too far.
There are no rights and wrongs... the whole design has to be considered as a whole.
Here is where i am lost ... if the fft is good does this not imply that the bjt will work ?
The bjts i have in mind are commonly used as drivers for output stages in high level power amps
So they should drive an input stage of a power amp without any problem as well
a much easier task i guess
Actually i have an amp insanely low at 5K .... but i would be willing to put it in the garbage bin.
For me the preamp makes the sound ... just a honest and robust power amp will do the rest.
As i said already when there is the review of a cheap couple, let's say Rotel, it is almost always the preamp that fails to convince
Usually the sound is flat and hard
The evidence is that good solid state preamp are rare and usually hugely expensive.
But they make the sound ... they give the 3d effect for one.
Thanks and regards, gino

Last edited by ginetto61; 3rd July 2014 at 06:37 PM.
  Reply With Quote
Old 3rd July 2014, 07:46 PM   #29
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Just looking at the fft isn't always going to guarantee the best (sonic) performance because there will always be something "different" that you perceive as "better".

I think you are going to have to trust your ears and try building a few different designs tbh. All the single ended designs (as opposed to long tailed or differential input stages) tend to have that "musical" and non fatiguing sound to them in my experience. And it seems like that is what you like too.

Driving 5k to a few volts is no big deal, its only a milliamp or so at the kind of levels a power amp input would need.
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
  Reply With Quote
Old 3rd July 2014, 08:37 PM   #30
diyAudio Member
 
Join Date: Sep 2009
Location: Stavanger (NORWAY)
Quote:
Originally Posted by Mooly View Post
Just looking at the fft isn't always going to guarantee the best (sonic) performance because there will always be something "different" that you perceive as "better".
I think you are going to have to trust your ears and try building a few different designs tbh. All the single ended designs (as opposed to long tailed or differential input stages) tend to have that "musical" and non fatiguing sound to them in my experience. And it seems like that is what you like too.
Driving 5k to a few volts is no big deal, its only a milliamp or so at the kind of levels a power amp input would need.
Thanks again and i think you are perfectly right
I like for instance the BOZ that is single ended and i think has indeed some 2nd order distortion.
I have seen that there are some models of drivers in the Cordell's file
They are to126 type .. i will start with one of them i think
Thanks a lot again. Kindest regards, gino
  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
new spice models available Joel Tubes / Valves 32 10th July 2013 06:30 PM
In praise of: Cordell SPICE models of BJTs Mark Johnson Software Tools 3 26th September 2011 03:42 PM
Looking for SPICE models of power BJTs Jorge Solid State 9 21st July 2005 02:13 PM
Spice models JoeBob Solid State 18 25th April 2002 03:34 PM


New To Site? Need Help?

All times are GMT. The time now is 05:48 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