Availability of parts (was: Compensated Exp generators)

[The Old Crow]

On Thu, 1 Jun 1995, Rick Jansen wrote:

> 
> In message <2FCDE89E at MSMGATE.M30X.NBG.SCN.DE> JH writes:
> 
> About discharging a capacitor with MOSFET/JFET/HC4066 switches
> and Rds(on):
> 
> > I know that there is a difference between
> > '4016 and '4066, and I thought the standard CD4066 had 60 Ohms.

  I've found the Signetics DG308A/DG309 series of analog switches to be 
the most suitable in low-impedance switching.  They are quite common, and 
have the advantage of working at +15/-15V.

> > But if You want ultra-fast discharge, why not go for the thyristor
> > solution? (Thyristor made of 1 NPN + 1 PNP transistor. I haven't done
> > this myself, but it seems to work fine in many Roland and Korg VCOs.)
> 
> I'll look into this. I think I've seen such a circuit in The Art of
> Electronics, but hadn't given it more thought.

  For an example, the same National Semiconductor appnote (AN-299) that
demonstrates the LM3046 temperature-controlled exp converter also shows a
sawtooth VCO where the firing circuit is this PNP-NPN pair wired as a
thyristor trigger.

  -Scott

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