[sdiy] Old unique VCO - T?==?windows-125 6?Q?une drifting problem - adapting a servo c ?= =?iso-8859-1?Q?onversion‏

Idris El Kasri idris1er at hotmail.com
Sat May 12 01:08:45 CEST 2012


  Ok, thanx for helping me to understand that!
So 2 solutions :
  1) Thermostating, something like the Minisonic as advised by Allan Needham
  2) Using a matched pair

I apologize for my poor level of knowledge in electronics but I have great difficulties in understanding how to put a matched pair in my schematic :
http://img826.imageshack.us/img826/646/vco2shematic.jpg

  I ve tried that unsuccesfuly :
http://img337.imageshack.us/img337/1861/servow.jpg
  Then that, no results either...
http://img26.imageshack.us/img26/1861/servow.jpg


----------------------------------------
> Date: Fri, 11 May 2012 14:45:48 -0600
> To: dixon at mail.ubc.ca; rtarcan at superonline.com; Synth-diy at synth-diy.org
> From: ijfritz at comcast.net
> Subject: [sdiy] Re: Old unique VCO - T?==?windows-125 6?Q?une drifting problem - adapting a servo c ?= =?iso-8859-1?Q?onversion‏
>
> At 02:33 PM 5/11/2012, David G Dixon wrote:
> > > The tempco resistor only compansates VT parameter of
> > > transistor but not IEBS of transistors( Emetor -Bas
> > > saturation current of transistors) witch drifts too much with
> > > ambient temperature. But heating the transistors as used in
> > > uA726 compansates both...
> >
> >Actually, if the transistors are relatively well matched and in thermal
> >contact (as they would be in a dual package), then wiring them in
> >common-emitter mode automatically compensates for I_S.
>
> Correct. The I_S temperature dependence can be removed either by using a
> matched pair or by thermostating. There are designs that do both, for some
> reason, and this can be confusing. But as pointed out above, if you use a
> matched pair you additionally need to add the tempco resistor to compensate
> the 1/T term in the exponent. This is not needed with thermostating.
>
> Ian
>
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