Tempco's

[Grant Richter]

> 
> 
>    I was just curious -
> 
> 
>     If there was a standard value of tempco resistor to buy, what would
be the
> best comprimise? I know it's supposed to be 3000 ppm/C, but positive or
> negative?  Also, what value 100ohm 1K ???? What is the most useful?
> 
> 

2K (2000 ohm) 3300 ppm is a standard value for 100K input resistor. This
reduces to 20mv / volt input which is then attenuated by a 100 ohm trim pot
in series with a 390 ohm resister to get to 18 MV / volt input which is
nominal for a 2:1 change in transistor current. The other option is to
thermostate the transistor pair used for the conversion. National
application note 299 shows a circuit. This is actually cheaper than a
tempco at US$4 and Moog and Serge felt it was the better solution. Although
it's mostly a matter of taste.