[sdiy] Temperature stable lin-exp converter with a CA3086 or CA3046

[Ian Fritz]

At 08:47 PM 5/27/2002, Scott Bernardi wrote:
>One reason might the value of the tempco available.

Yeah, I guess so. It probably takes a special order to get a 2k unit these 
days.

>To divide 1v down to the
>approximately 18mV needed is a ~55:1 divider. With 100K inputs, that means 
>about
>1.8K.  If you have 2K tempcos, that works great because you just use a 
>multiturn
>trimpot after the opamp to adjust the V/octave.  I got a bunch of 1K 
>trimpots from
>EFM; I wanted to keep the 100K inputs (I guess I could have gone to 
>49.9K), so I
>used the tempco in the voltage divider.

With a 1k tempco in the voltage divider you will have a 1k source driving 
the converter. The base current will then load the source a bit. Maybe not 
a big deal, especially with an expensive high beta converter transistor.

   Ian