temperature compensation.

[Grant Richter]

You can also then use the 3500ppm tempcos that are (were?)
in the Farnell catalog. I bought 25 of these by mistake (at $3.25 ea).
This is a 1K 3500ppm POSITIVE tempco. You can put them in
the high side and use a 18 ohm on the low side. You will have
to buffer them with an op-amp of course.

----------
> From: Rene Schmitz <uzs159 at uni-bonn.de>
> To: Paul Maddox <space_banana at hotmail.com>; synth-diy at mailhost.bpa.nl
> Subject: Re: temperature compensation.
> Date: Sunday, August 01, 1999 8:20 AM
> 
> At 10:29 27.07.99 GMT, Paul Maddox wrote:
> >Dear all,
> >
> >  Ive been toying with tempature compensation of VCO's.. for most
circuits 
> >we need a NTC 3300 PPM resistor..
> >
> >  If you look at the ASM one we have this
> >
> >  OPAMP> -----VVVV----+------ base of transistor
> >                      >
> >                      < NTC resistor
> >                      >
> >                      |
> >                      0v
> >
> >  Why couldnt you simply swap the two around and use a PTC, these seem
to be 
> >more common, ie....
> >
> >  OPAMP> -----VVVV-----+---- Base of transistor
> >             PTC res   |
> >                       >
> >                       <  Fixed value res
> >                       >
> >                       |
> >                       0v
> >
> >  Am I missing something obvious as to why this couldnt be done?
> >
> >  Paul
> 
> Hi Paul and List
> 
> Above you swapped NTC for PTC. The tempco resistor in the ASM is a PTC. 
> Now, I've sucessfully done what you propose here. I found that all the
> cheap NTC resistors have an exponential temperature coefficient of -4%/K.
> This value depends 
> on the band gap of the used semiconductor material, which seems to be the
> same for all the cheap NTCs. Si (?). When you replace the 54k/1k(tempco)
> divider with a 51k+4.7k(NTC)/1k divider one gets pretty good temperature
> cancellation. 
> (Theres a rule of thumb for the series resistor: 11 times the value of
the
> NTC because 4%/12=0.33%)
> Of course the compensation is not exact at all temperatures, because of
the
> exponential temperature dependance. But I needn't. You can even find an
> example, on my website. I've compensated one of my VCO 2s with  this
> divider scheme. Works nice. I did also simulations which showed that good
> stability can be archeived arround room temperature.
> 
> 
> Bye
>  René (aka Tempco Geek)
> 
>              ,   : (uzs159 at uni-bonn.de)
> |") [" |\ | ["   : (http://www.uni-bonn.de/~uzs159)
> |"\ [_ | \| [_   : (http://members.xoom.com/Rene_Schmitz)
>