[sdiy] CA3086 and CA3046?

[patchell]

    I don't know why Tom Oberheim used the T-divider in the reference leg of the
exponential converter, but I used to use one for the simple reason I could not
afford the the then rather expensive FET opamps.  Using the  T divider would give
me an equivalent large resistance, without the problem of the bias current from
the BJT opamp getting in there and messing things up.

    -Jim

René Schmitz wrote:

> At 17:46 17.08.01 +0200, jh. wrote:
> >> That depends on the rest of the circuit - when the output collector of the
> >> expo runs into a virtual ground node for example, you'd be safe. The
> >> transistors would never see more than -15volts. But when the collector is
> >> going to a higher potential, the 20volts for Uceo might be exeeded.
> >
> >I suspect that's why Tom Oberheim used that strange T-divider instead of
> >a single resistor for the ref current in the SEM's expo converters.
> >Normally, virtual GND, but you can never know.
>
> I've wondered about that, too. But then pin 13 is tied to -1.5V via that
> 1k/10k divider, so even when the collectors would hit 15V there would be a
> margin of 3.5V.
>
> >Others were more adventurous. I think ARP has violated the pin13-most-
> >negative-rule once. And in general, they knew what they were doing:
> >Some of the most impressive circuits built from 3086 BJT and
> >4007 MOSFET arrays.
>
> Hmmm. In that circuit (if the Quadra is what you mean) the pin 13 is
> open. Now, what would happen if one simply can leaves pin 13 open?
> As I see it the insulating diodes between any two collectors would be
> back to back. And no current could flow.
> Would we only need to tie pin 13 to the lowest potential if the fifth
> transistor is actually used in the circuit?
> That could be an alternative interpretation of the datasheet.
>
> Cheers,
>  René
>
> --
> uzs159 at uni-bonn.de
> http://www.uni-bonn.de/~uzs159
>
>

--
 -Jim
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