Vbe mult + thermistor - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 1st August 2006, 12:11 AM   #1
diyAudio Member
 
Join Date: Sep 2005
Default Vbe mult + thermistor

Okay, I know this topic has been discussed before, and I know that in the end, the basic Vbe multiplier comes out on top over a thermistor for voltage regulation as a function of temperature.

I don't *think* this idea has been discussed in other threads:
Why not use a thermistor inside of the Vbe multiplier?

==> Let us say we have your basic NPN multiplier with R1 on top (base to collector) and R2 on the bottom (base to emitter). How about replacing either one with a thermistor for sharper response, and then have the thermistor itself (as opposed to the transistor) mounted on the heatsink?

If you have a positive tempco thermistor (resistance increases with temp), then use it to replace R2 (the thermistor gets hotter, so R2 increases, thus reducing the 1 + R1/R2 product). If you have a negative tempco thermistor, use it to replace R1 (same reasoning as above).

I read in Douglas Self's book a method to increase the tempco of the basic Vbe multiplier (so I am assuming that a higher tempco is better than a lower tempco). I figured with this method, the tempco of the overall Vbe multiplier is a factor of the B constant (which can be made very high, depending upon the thermistor used).

Ideas? Am I off my rocker?
  Reply With Quote
Old 1st August 2006, 07:06 AM   #2
diyAudio Member
 
Join Date: Nov 2004
Location: Florida
Make a small test amp, and try it.

It sounds like a good idea for controlling the bias. I've never tried it myself, but experiment with it to see what happens. I'm guessing you may need to use a POT with the thermistor for testing until you figure out what resistor to use.
  Reply With Quote
Old 1st August 2006, 01:54 PM   #3
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
if you select a high value thermistor and use it in parallel to one of your resistors then you can get partial compensation.
You may be able to trim the thermistor effect by adding another resistor in series.
A pot here might allow you to investigate the tempco.

I think a thermistor on it's own will vary far too much, even a low value NTC varies more than 10:1 over quite a small range of temp from ambient.

Have you tried putting a thermistor bridge on the inputs to an opamp and measure what happens? just putting your hand near absorbs heat radiation and blowing gently puts the output meter offscale.
__________________
regards Andrew T.
  Reply With Quote
Old 1st August 2006, 07:45 PM   #4
latala is offline latala  United Kingdom
diyAudio Member
 
Join Date: Feb 2006
Location: staffordshire
I have done this myself on Three differant amps with satisfactory results !
You still have to be carefull where you place the thermistor for optimum tracking !
You can easily overcompensate and end up with the bias going down when the output devices warm up. Sometimes I wonder if we are biasing correctly anyway as the heat has to come out of the device before we compensate. I believe that there are devices out there that have a temperature tracking diode on the die of the output device. Bet that would track well!

regards Trev
  Reply With Quote
Old 1st August 2006, 09:06 PM   #5
diyAudio Member
 
destroyer X's Avatar
 
Join Date: Feb 2004
Location: Recife - Brasil Northeast
Default Good rtarbel...post some sketch, good idea, let me know

How you are doing this circuit.... very good.... will have a better temperature sensor....very good!

regards,

Carlos
__________________
Brazilian Airways Christmas Message to all of you!
https://www.youtube.com/watch?v=HFGM5K-WWnY#t=135; Unbeatable!; https://www.youtube.com/watch?v=u4tGBiqMnzQ
  Reply With Quote
Old 1st August 2006, 09:16 PM   #6
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Hi rtarbell,
It's a good idea. A few amps have done things that way. This becomes more useful as your heatsink size goes down. Andrew has the right idea for minimizing your temperature co-efficient.

-Chris
  Reply With Quote
Old 1st August 2006, 09:59 PM   #7
latala is offline latala  United Kingdom
diyAudio Member
 
Join Date: Feb 2006
Location: staffordshire
I once designed a power supply that used the LM318K steel regulator chip . This required a very tight voltage of 36 volts and despite the perceived claims found that the output varied by about 0.5 volt over a temperature range of 10-30 deg using the design sheet schematics, however by using a neg value thermistor (10k @ 20c ) in parrallell with a I think 470 resistor I managed to bring the variation down to about 25 my . Over a 100 units were produced and used with very good results!!
I say this not as a boast, but as encouragement to others as this is a valid compensation technique!
However there is and always will be time lag re the compensation and prefer to attempt to design out drift in the first place
Regards trev
  Reply With Quote
Old 1st August 2006, 10:08 PM   #8
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Hi Trev,
Try that with high voltage regulators. It's a requirement. The temperature drift can easily be over 10 V just due to your reference heating. 100 V zeners drift a lot!

-Chris
  Reply With Quote
Old 1st August 2006, 10:33 PM   #9
latala is offline latala  United Kingdom
diyAudio Member
 
Join Date: Feb 2006
Location: staffordshire
You are dead right ! That Is why I used the term perceived
the specifiers can be very deviouse and know that the die will self heat and move the ref source on the chip !!!
I was in trouble on that project untill I came up with the thermistor compensation. But to be Honest I had seen thermistors used by Leak and also Trio / kenwood in some of their older amps. Clever these old boys when you think what they had to work with !

Regards Trev
  Reply With Quote
Old 1st August 2006, 10:48 PM   #10
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Hi Trev,
I always respect what an engineer before me has accomplished. You should never be in a rush to redesign an old tube amp.

Some of the newer designs? Yeah, rip away.

-Chris
  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
Thermistor help luvtheoldschool Parts 5 4th February 2008 01:56 PM
help with ntc thermistor jarthel Parts 4 1st October 2006 05:57 PM
FS or Trade: ADB Mult!wav Digital I/O ISA Card theAnonymous1 Swap Meet 2 26th March 2006 06:46 PM
thermistor larryg Solid State 15 21st October 2004 04:40 PM
Attaching mult. headphones to amp speaker out Cradle22 Solid State 6 5th July 2003 08:23 PM


New To Site? Need Help?

All times are GMT. The time now is 03:31 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