[sdiy] Expo converter tempco placement

[Jim Patchell]

Well, in my opinion, the best place for the Tempco is in the garbage 
can....but, that is another story.

There are two main differences.

1. In the case of the tempco in the feedback loop, you get Vo = R(t)/Ri 
* Vin, which is good.  In the case of the tempco at the base of the expo 
transistor, you get Vo = Vi * R(t) / (Ri + R(t)), which is not so good.  
The later will have a slight error in compensating the drift.

2. However, when the tempco is in the feedback loop, because of its 
size, the input offset error of the opamp becomes rather significant.

So it comes down to which compromise you want to make.  Modern opams 
have much lower offset error, making number two a lot less significant.

-Jim

On 7/9/2014 8:40 AM, Justin Owen wrote:
> Hello,
>
> Can anyone give any insight into the differences between the two main locations for tempco placement in expo converter circuits?
>
> Seems like the schems I have are divided fairly evenly between those who use a tempco (generally 2K) in the feedback loop of the CV scaling op amp that feeds into the base of the expo pair and those that use a tempco (generally 1K) as part of a voltage divider at the output of the CV scaling op amp.
>
> Secondly, if you were using a tempco int he feedback loop of the CV scaling op amp - would there be any problems using that tempco in parallel or series with another standard R to achieve a certain R value?
>
> Thoughts?
>
> Thanks!
>
> - Justin
>
>
>   
>
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