SV: [sdiy] QVCO schemo up

[Ian Fritz]

At 01:19 PM 11/12/05, harrybissell wrote:

> > >let me say that I have never seen an LM13600/13700 that did not
> > >have huge offset problems when I activate the diode bias
> >
> > Maybe I'm not clear on what you are saying here.  Do you mean (a) you have
> > to make a big change in offset correction when the diodes are turned on or
> > (b) that the change in bias with Iabc gets much worse (after re-trimming)?
> >
>
>I'm saying "A".   I usually lay out al my OTA stuff to allow the diode bias
>resistor to be added.  Adding it blows the offset WAY out.

OK, no argument with that at all.  And changing the impedance of the 
driving source also upsets the offset adjustment.  But in the end, I still 
say it is "B" that matters.  (You have to trim anyway).


>I could see that the manufacturer's method has some advantages (low component
>count) but if the trimpot fails in my case, the offset gets a bit 
>untrimmed. If
>the
>trim fails in their case, the circuit will become totally non functional, 
>na ???

Fair point.

   Ian