[sdiy] Tempco adjuster idea

[Ian Fritz]

Hi Magnus --

At 05:47 AM 4/27/2003, Magnus Danielson wrote:
> >
> > Keep in mind that this is already an approximation for the temperature
> > dependence of an idealized pn junction. Once you get the diffusion
> > resitances and other second order stuff in, things start to look
> > different again. The rest of the circuit will have it's own idea about
> > temperature dependence as well, especially noticeable if you're not
> > careful with the type of resistors.
>
>Things like bulk resistance can be handled separately by fortunately much 
>smaller
>alterations to the curcuit. However, the bulk resistance compensation 
>usually seen
>out there only handles part of the bulk resistance problem, as I have 
>pointed out
>before.

Correct, but it is pretty close as long as you use a decent transistor. 
Also, remember that Rene came up with a clever improvement to the usual Rbe 
scheme that should take care of your objections.

>So, handling the temperature compensation which comes from the fundamental 
>expression
>kT (which is a quantum mechanic energylevel) is still a valid study.

Actually, the kT comes from classical statistical mechanics 
(thermodynamics), but thanks for the support!  :-)

> > The TKR is defined as 1/Ro*(R(T)-Ro)/(T-To), which is why you always
> > need to know what Ro and To is. This is different from the above
> > differential definition except around To and also different from
> > (dR/dT) except for linear temperature dependence.
>
>True linear curve may not exist really, but it may be accurate enought in 
>the range
>we are discussing.

That's what I want to get at ultimately.  With the correct tempco at room 
temperature how do the remaining nonlinearities and drift stack up vs. 
whatever errors you get with active compensation (like the spectacular 
Patchell result)?

>However, the tempco-less discussions we had before seemed very interesting 
>and with
>the amount of hardware used to compensate the curve of tempcos we are not 
>too far off
>in hardware to start with.

Agreed. And I know Jim is working on simplifying his design. Now if I could 
just understand how it works!

> > > So to correct for an incorrect coefficient one just needs to make a
> > > circuit that cancels the B term.  This looks easy to do, at least on
> > > paper, and the only drawback is that it takes several amplifiers
> > > instead of just one to condition the control voltage inputs.
> >
> > No, you still have to fight both coefficients. The offsets introduced
> > by B can be canceled by trimming at a single temperature. Then when
> > you find that the compensation fades as you go away from that
> > temperature, that means that A is incorrect. You can trim A towards
> > zero rather easily with passive components, but not the other way
> > around. BTW, Conrad has a bunch of 3900ppm/K temp sensors from Heraeus
> > that are almost reasonably priced.
>
>Why do I start to consider ovenized oscillators as a fair deal?

I have no idea.  :-)

Are you talking about putting the whole circuit in an oven, or just the 
usual heat-the-converter-transistor business?

> > > I hope to try this idea out in the near future so that I can use
> > > the  3200ppm/K units that KRL sold me.
> >
> > Heat up your expo converter to 40°C for the trim and save the opamps
> > for something more useful. :-)
>
>My point. Ovenized oscillators is a known trick of the trade and when done 
>properly
>you can acheive *really* stable temperature, enought to make that ~ 3300 
>ppm/C dampen
>down to way below other sources of drift. This is done for crystal 
>oscillators on a
>regular basis.

It could be the ultimate solution, although a bit inelegant.  One thing to 
watch for would be whether the high temperature degrades any component 
performance. You might want to get rid of all the FET-input op amps, for 
example.

   Ian