[sdiy] CA3080 switch driving voltage

[jhaible]

> That was my old work on my ultrasonic VCO (Electronotes #112).

Oh yes, there it is!


> I found a 60 mV drive to give the best tracking.  I'm still waiting for a
> good explanation of this effect.

60mV - much lower than I expected.
Doesn't this mean that there is a temperature drift from the diff pair
still working in the "almost linear" area?
But anyway - if saturation must be avoided, then this is the way to go.
Probably compensating for the tempco somewhere else. (I remember
there was another EN article with diodes setting the thresholds of
the schmitt trigger - maybe this was exactly for that reason.)

Thanks for pointing this out. All my previous triangle cores show
the effect you describe in EN#112. Not tracking ore than 3 octaves
accurately, and HFT trimmers won't help. This time I'll try the 60mV.

(I hope these will be the same for the LM13600; I need 2 tracking
OTAs, so I have to use the 13600.)

JH.





 Maybe now that I have a better 'scope I
> could characterize it better.
>
> Please let me know if you figure it out.  It seems that the chip doesn't
> like the input transistors to be saturated, for some reason.
>
>    Ian
>
>
> At 01:24 PM 9/15/2003, jhaible wrote:
> >I remember I've seen this somewhere (Electronots?), but
> >I don't remember _where_.
> >
> >If you build a triangle VCO core,
> >opamp integrator, 3080 providing the integration current,
> >schmitt trigger to switch the 3080's current direction,
> >
> >what is the optimal voltage and impedance to drive
> >the 3080 input, in order to get the best linearity
> >of frequency vs. control current ?
> >
> >At first, you might think maximum overdrive with
> >low impedance would be best (like 5V and 0 Ohm)
> >to get immediate switching (no RC time constant
> >with parasitic C's), but apparently such a configuration
> >is "too much" and degrades the linearity of
> >Iabc -> Iout operation. (Yes, speaking of switching!
> >Maybe base current? Maybe too much common
> >mode signal? Don't know ...)
> >
> >So, where is the optimum? I vaguely remember
> >a resistance of a few hundred ohms, but I
> >don't remember the voltage at all. Certainly
> >more than 100mV, certainly less than 5V.
> >
> >JH. (Working on an improoved version
> >         of the FS-1)
>
>