[sdiy] VCO tempco R locataion

[Ian Fritz]

At 02:35 PM 7/30/2010, Stewart Pye wrote:
>I'm planning on using OPA2134 op amps that have a max Vio of 2mV.
>
>I'm looking at your Sawtooth VCO design at the moment and planning on 
>basing my circuit on that, but using REF02 for a 5V reference voltage, and 
>possibly using the tempco resistor in the voltage divider to the base of 
>the expo transistor.

Hi Stewart --

Sounds good -- that posted circuit is a bit long in the tooth now!

I agree with using 5V references.  They don't even make the low drift 
version of the 6.9V unit anymore.

Nowadays I use an OPA 2227 for the expo summing amp and servo.  Better 
specs, but I'm not sure I can really notice any difference from the 
OPA2134. You might not want to go that route if you use the voltage-divider 
method, though, because of the slew-rate limitation.

   Ian