SV: [sdiy] QVCO schemo up

[Ian Fritz]

At 07:52 PM 11/11/05, 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)?

For me, (a) is certainly true, but not a concern and (b) sometimes happens 
but isn't always a lot worse.  But again, these chips are a total crap 
shoot, as far as I am concerned.  I learned why making discrete 
versions.  So now I always pretest any OTA I use.  I built test circuits 
and ran dozens of chips through them, recording all the results on paper.


>I'd add that I usually use equal value bias resistors... then inject a small
>offset voltage through a high value (megohms) resistor.

You waste a resistor?!?!?  MEGA Ohms down the drain?!?!?  When you only 
need to use 100 or 200?!?!?  Gasp!!!!!


>I don't usually use the trimpot as Ian showed... (had bad luck with that
>once).
>Is that method any better at trimming the offset ???

I don't see where it would make a difference.  I just lazily follow the 
manufacturer's suggestion, and conserve as many Ohms as possible.

   Ian